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PORN A OF GROLOGY

A Semi-Quarterly Magazine of Geology and

Related Sciences.

EDITORS T. C. CHAMBERLIN

Rk. D. SALISBURY, CeReAVEAIN EUISiIE.

Geographic Geology. Pre-Cambrian Geology. To 1a BID IUNIGSy, Ca IDs WAIL COUT.

Petrology. Paleontologic Geology. IR, ANs 185 IPIBINIROSIE, lire We daly Jal(OIIMUES,

Economic Geology. Archeologic Geology.

GEORGE BAUR,

Vertebrate Paleontology.

ASSOCIATE EDITORS

SIR ARCHIBALD GEIKIE, JOSE EEE CONME Great Britain. University of California. H. ROSENBUSCH, Go. IX, (GAULIBIGIN AC, Germany. Washington. CHARLES BARROIS, H. S. WILLIAMS, france. Vale University. ALBRECHT PE NGK, J. Go BRANNEIRS Austria. Leland Stanford, Jr. University. HANS REUSCH, G. H. WILLIAMS, Norway. Johns Hopkins Untversity. GERARD DE GEER, ll, @ IRNWSSIBICIL, Sweden. University of Michigan. GEORGE M. DAWSON, OF AGE DE RBYe Canada. Brazil.

VOLUN gal -tnsonian last

S 6 1893 ational Museu: 7 Gr CAG@ . —

Che Anthersityp Press of Chicago

1D), c. HEATH & Con Disectors.

be ‘i As ee t iM y i , . hoe

CONTENTS OF VOLUME /

NUMBER I. ON ‘THE PRE-CAMBRIAN ROCKS OF THE BRITISH ISLES. Sir Archibald Geikie. P ‘ ARE THERE TRACES OF GLACIAL MAN IN THE TRENTON GRAVELS? W. H. Holmes.

GEOLOGY AS A PART OF A COLLEGE CURRICULUM. H.S. Williams.

THE NATURE OF THE ENGLACIAL DRIFT OF THE MISSISSIPPI BASIN. T. C. Chamberlin. ; : ‘

STUDIES FOR STUDENTS: Distinct Glacial Epochs and the Criteria for their Recognition. Rollin D. Salisbury.

EDITORIALS. REVIEWS: On the Glacial Succession in Europe, James Geikie; by Rollin D. Salisbury. . : - : 0 0 5 . 6 :

ANALYTICAL ABSTRACTS OF CURRENT LITERATURE: The Sub-Glacial Ori- gin of Certain Eskers; William Morris Davis. Studies in Structural Geology; Bailey Willis. The Catskill Delta in the Post-Glacial Hudson Estuary; William Morris Davis. Geological Survey of Ala- bama— Bulletin 4; C. Willard Hayes. The Correlation of Moraines with Raised Beaches of Lake Erie; Frank Leverett. The Climate of Europe during the Glacial Epoch; Clement Reid. On the Glacial Period and the Earth Movement Hypothesis; James Geikie.

ACKNOWLEDGMENTS.

NUMBER II. AN HISTORICAL SKETCH OF THE LAKE SUPERIOR REGION TO CAMBRIAN Time. C.R. Van Hise. THE GLACIAL SUCCESSION IN OHIO. Frank Leverett. TRACES OF GLACIAL MAN IN OHIO. W. H. Holmes. THE VOLCANIC ROCKS OF THE ANDES. Joseph P. Iddings.

ON THE USE OF THE TERMS POIKILITIC AND MICROPOIKILITIC IN PETROG- RAPHY. George H. Williams. . . é 5 5

STUDIES FOR STUDENTS. The Making of the Geological Time-Scale. H. S. Williams. 5 ; : ; ; : : : :

EDITORIALS. ili

95-100

101

113 129

147 164

176

180 198

iv : CONTENTS OF VOLUME 1,

ANALYTICAL ABSTRACTS OF CURRENT LITERATURE. The Age of the Earth; Charles King: The Age of the Earth; Warren Upham. Continental Problems; G. K. Gilbert. Measurement of Geological Time; T. Mellard Reade. Recent Archeological Explorations in the Valley of the Delaware; Chas. C. Abbott. The Drainage of the Jura; August F. Foerste. Deep-Sea Sounding; Capt. A.S. Barker. Obser- vations and Experiments on the Fluctuations of the Level and Rate of

Movement of Ground-Water; Franklin H. King. 2 : 3 . 202-208 ACKNOWLEDGMENTS. . ‘ 5 ; ; ; : : ; ; so 209 NUMBER III.

MALASPINA GLACIER. Israel C. Russell. . : y ; : : 6 219 THE OSAR GRAVELS OF THE Coast OF MAINE. George H. Stone. . 246

THE HoRIZON OF DRUMLIN, OSAR AND KAME FORMATION. T. C. Chamberlin. : : :

A CONTACT BETWEEN THE LOWER HURONIAN AND THE UNDERLYING GRANITE IN THE REPUBLIC TROUGH, NEAR REPUBLIC, MICHIGAN. Henry Lloyd Smyth. . 0 . 6 : : é 0 : 268

A PLEISTOCENE MANGANESE DEPOSIT, NEAR GOLCONDA, NEVADA. R. A.

255-

F. Penrose, Jr. ; : : B 275 STUDIES FOR STUDENTS: The Elements of the Geological Time-Scale. H. 8S. Williams. : : ; i ; ‘ : 283

EDITORIALS. j 3 5 4 é ea : : : 296

REVIEWS: Monographs of the U.S. Geological Survey, vol. XVII. The Flora of the Dakota Group; Leo Lesquereux, by David White. Cretaceous Fossil-Plants from Minnesota; Leo Lesquereux, by F. H. Knowlton. On the Organization of the Fossil-Plants of the Coal-

Measures; W. C. Williamson, by F. H. Knowlton. : : F . 300-303 ANALYTICAL ABSTRACTS OF CURRENT LITERATURE; Summary of Current

Pre-Cambrian North American Literature; C. R. Van Hise. : 5 304 ACKNOWLEDGMENTS. 6 : : : . : 5 ‘ 0 ; 315

NUMBER IV

ON THE TYPICAL LAURENTIAN AREA OF CANADA. Frank B. Adams. . 325 MELILITE-NEPHELINE-BASALT AND NEPHELINE-BASANITE FROM SOUTHERN

BBE XAG Av © SATs 6 ; : 0 . . 6 : : 341 SomME DyNAMIC PHENOMENA SHOWN BY THE BARABOO QUARTZITE

RANGES OF CENTRAL WISCONSIN. C.R. Van Hise. ; 6 ; 347 THE CHEMICAL RELATION OF IRON AND MANGANESE IN SEDIMENTARY

IROCIKG, IRS ING IR, eterno Ss iro) > f ; : : : : ‘ 356 SoME RIVERS OF CONNECTICUT. Henry B. Kimmel. . j 5 : 371

STUDIES FOR STUDENTS: Geological History of the Laurentian Basin. Israel C. Russell. : ; : 6 6 : : ‘ : ; 394

EDITORIALS. d : : . : . ¢ ; 9 ; a ; 409

CONTENTS OF VOLUME I. Vv REviEws: Crystalline Rocks from the Andes; Untersuchungen an altkrys- tallinen Schiefergesteinen aus dem Gebiete der argentinischen Republik ; B. Kiihn. Untersuchung argentinischen Pegmatite, etc.; P. Sabersky. Untersuchungen an argentinischen Graniten, etc.; J. Romberg, by - George H. Williams. The Mineral Industry, its Statistics, Technology and Trade in the United States and other Countries ; Richard P. Roth- well, by R. A. F. Penrose, Jr. : P 6 : : ses . 411-414 ANALYTICAL ABSTRACTS OF CURRENT LITERATURE: A New Teeniopteroid Fern and its Allies; David White. Rainfall Types of the United States; Gen. A. W. Greely. Geographic Development of the Eastern Part of the Mississippi Drainage System; Lewis G. Westgate. On a New Order of Gigantic Fossils; Edwin H. Barbour. The Vertical Relief of the Globe; Hugh Robert Mill. : : 5 . 419-422 ACKNOWLEDGMENTS. 423 NUMBER V. - THE Basic MASSIVE ROCKS OF THE LAKE SUPERIOR REGION. W.S. Bayley. 433 NOTES ON THE STATE EXHIBITS IN THE MINES AND MINING BUILDING AT THE WORLD’S COLUMBIAN EXPOSITION, CHIcAGo, R. A. F. Penrose, Jr. : : 457 THE Las ANIMAS GLACIER. George. H. Stone. 471 STUDIES FOR STUDENTS: Conditions of Sedimentary Deposition; Bailey Willis. F : F : : : 476 EDITORIALS. 521 REVIEWS: Correlation ee Archean and a Algona: C. R. Van Hise, by Rollin D. Salisbury: 525 ANALYTICAL ABSTRACTS OF CURRENT LITERATURE: eee of Current Pre-Cambrian North American Literature. 0 : C : 532 NUMBER VI. THEORY OF THE ORIGIN OF MOUNTAIN RANGES. Joseph Le Conte. 542 ON THE MIGRATION OF MATERIAL DURING THE METAMORPHISM OF ; Rock Masses. Alfred Harker. 574 THE CORDILLERAN MeEsozoic REVOLUTION. Andrew C. Lawson. 579 SKETCH OF THE PRESENT STATE OF KNOWLEDGE CONCERNING THE BASIC MASSIVE ROCKS OF THE LAKE SUPERIOR REGION. W.5S. Bayley. 587 A STUDY IN CONSANGUINITY OF ERUPTIVE Rocks. Orville A. Derby. 597 A DISSECTED VOLCANO OF CRANDALL BASIN, WYOMING. Joseph P Iddings. : : 0 . , 606 NOTES ON THE LEAD AND ZINC DEPOSITS OF THE MISSISSIPPI VALLEY AND THE ORIGIN OF THE ORES. Arthur Winslow. 612 EDITORIALS. 620 REVIEWS: Eruptive Rocks of Montana; Waldemar Lindgren. A Sodalite- Syenite and other Rocks from Montana; W. Lindgren. Acmite- Trachyte from the oe Mountains, Meat aniae th E. Wolff, a Ieee P. Iddings. 634

vi CONTENTS OF VOLUME I. NUMBER VII. GEOLOGIC TIME, AS INDICATED BY THE SEDIMENTARY ROCKS OF NORTH AMERICA. Charles D. Walcott.

ON THE ORIGIN OF THE PENNSYLYANIA ANTHRACITE. John J. Stevenson.

THE Basic MaAssiIvE ROCKS OF THE LAKE SUPERIOR REGION. W. S. Bayley. ° 6 0 6 . ; ;

639 677

688

ON THE GEOLOGICAL STRUCTURE OF THE MouNT WASHINGTON Mass _

OF THE TACONIC RANGE— PlatesIII, 1V. Wm. H. Hobbs. EDITORIALS.

REVIEWS : Correlation Papers, The “ Newark” System; Israel Cook Russell, by Wm. M. Davis. Text-Book of Comparative Geology; E. Kayser, Ph.D., by Rollin D. Salisbury. lowa Geological Survey, vol.I. First Annual Report; Samuel Calvin, State Geologist, by C. H. Gordon.

NUMBER VIII.

THE SUPPOSED GLACIATION OF BRAZIL. John C. Branner. CAUSES OF MAGMATIC DIFFERENTIATION. Helge Backstrom.

THE GEOLOGIAL STRUCTURE OF THE HOUSATONIC VALLEY LYING East OF MounT WASHINGTON; Plates V, VI, VI. Wm. H. Hobbs.

THE NEWTONVILLE SAND-PLAIN. F. P. Gulliver.

THE STRUCTURES, ORIGIN, AND NOMENCLATURE OF THE ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. F. Bascom.

STUDIES FOR STUDENTS: Genetic Relationships Among Igneous Rocks; Joseph P. Iddings. 0 :

EDITORIALS

9 ° .

REVIEWS: Recent Contributions to the Subject of Dynamometamorphism ; A. Heim. C. Schmidt, L. Milch,M.P. Termier, by George H. Williams. Text-Book of Geology; Sir Archibald Geikie, by Rollin D. Salisbury. Bodengestaltende eee der Eiszeit; Dr. Aug.

e Bohm, by Wm. M. Davis. . : : : :

ANALYTICAL ABSTRACTS OF CURRENT LITERATURE: Conditions of Appa- lachian Faulting; Bailey Willis and C. W. Hayes. Ueber Geroll- Thonschiefer glacialen Ursprungs in Kulm des Frankenwaldes; Ernest Kalkowski. :

ACKNOWLEDGMENTS.

717 737

- 740-747

O28) 773

780: 803

813

833 845

. 850-859

. 861-862

863

Tels,

lOWRKNAL OF GEOLOGY JANUARY- FEBRUARY, 1893.

ON “WIGhe, IPIR Is C ANUBIS ICAUN | IOC ICS) Op Wiss [eI I Sel USL Ia Sy,

Durine the last twenty years much has been written about the ‘ pre-Cambrian ” rocks of the British Isles. Unfortunately when attention began to be sedulously given to the study of these ancient formations, the problems of metamorphism were still a hundred fold more obscure than they have since become ; the aid of the microscope had not been seriously and system- atically adopted for the investigation of the crystalline schists, and geologists generally were still under the belief that the broad structure of these schists could be treated like those of the sedimentary rocks, and be determined by rapid traverses of the ground. We have now painfully discovered that these older methods of observation were extremely crude, and that the work performed in accordance with them is now of little interest or value save as a historical warning to future generations of geol- ogists. Geological literature has meanwhile been burdened with numerous contributions which remain as a permanent incubus on our library shelves.

It may serve a useful purpose at the present time in possibly aiding those who are engaged in the study of the oldest rocks of North America, if I place before them, as briefly as possible, the main facts which in my opinion have now been satisfactorily proved regarding the corresponding rocks of Britain, and if I indicate at the same time some of the more probable inferences in those cases where the facts, at present known, do not warrant a definite conclusion.

WOls is IN@>s ie

2 THE JOURNAL OF GEOLOGY.

It is obvious that in any effort to establish that a group of rocks is older than the very base of the sedimentary fossiliferous formations, we must somewhere find that group emerging from under the bottom of these formations. Until lithological char- acters are ascertained to be so distinctive and constant as to be comparable to fossil evidence for purposes of stratigraphical identification, we should not assume that detached areas of older rocks rising amid Paleozoic, Secondary or Tertiary formations are pre-Cambrian. We should, if possible, begin at the bottom of the Paleozoic systems and work backward, tracing each ~ successive system or group as these rise from under each other, until we arrive at what appears to be the oldest traceable within the region of Yobseryation, It is clear that ini) the \present state of knowledge we have no satisfactory means of identifying such successive systems in widely separated countries. All that can be attempted in the meantime is to ascertain the special types in each region, and to point out their general resemblances or contrasts to those of other regions. It is better to avoid con- fusion by refraining from applying the stratigraphical names adopted for the oldest rocks of one region to those of another geographically remote, though we may hope that eventually: it may be possible to work out the equivalence of these local names.

In the British Isles, by much the most important region for the study of the oldest rocks is to be found in the north-west Highlands of Scotland. The very basement strata of the Cam- brian system are there traceable for a distance of more than 100 miles, reposing with a strong unconformability upon all rocks of older date. They consist of dolomitic shales with Olenedlus, resting upon a thick group of quartzites, full of annelid tubes. One of the most remarkable features of these ancient strata is the persistence of their component bands or zones which, though sometimes only a few feet thick, can be traced throughout the whole tract of country just referred to. For the study of the pre-Cambrian rocks this is an important point, for we can be quite certain that even where fossil evidence locally fails, the

LE PRECAMBRIAN ROCKS, BRILLTSHT [SLES 3

same basement members of the Cambrian system are persistent and lie directly upon the pre-Cambrian series.

Lewisian Gneiss. Ever since the researches of Murchison and Nicol in the north-west of Scotland, it has been known that two distinct systems of rock underlie the quartzites to which I have just alluded. Murchison regarded the upper of these as of Cambrian age, while he assigned the unconformable quartzites and limestones above it to the Lower Silurian period. But the recent discovery of the Olenellus zone intercalated conformably between the quartzites and the overlying limestones may be regarded as proving that all the rocks which underlie the quart- zites and are separated from them by a strong unconformability must be pre-Cambrian. It is thus established beyond any reasonable doubt that two great pre-Cambrian systems of rock exist in the north-west of Scotland.

These two systems differ so entirely from each other that their respective areas can be defined with minute accuracy. The uppermost consists chiefly of dull reddish sandstones with con- glomerates, and especially towards their base in Rosshire, some bands of dark grey shale, the whole having a thickness of at least 8,000 or 10,000 feet, though as both the base and the top of the series are marked by strong unconformabilities, the whole original thickness of deposits is nowhere seen. As these rocks are well developed around Loch Torridon, they were named by Nicol the Torridon Sandstone—a designation which has more recently been shortened into ‘Torridonian.’’ The lower system is mainly composed of various foliated rocks which may be embraced under the general term ‘‘gneiss.’’ These masses pre- sent the usual characters of the so-called ‘fundamental complex” ‘‘Urgebirge,” or “‘ Archean Series”’ of other countries. The contrast between the thoroughly crystalline, gnarled, ancient- looking gneisses below, and the overlying, nearly horizontal Torridonian conglomerates, sandstones, and shales, which are largely made out of their debris, is so striking that every ob- server feels persuaded that in any logical system of classification they can not be both placed in the same division of the geolog-

4 THE JOURNAL OF GEOLOGY.

ical record. They are certainly both pre-Cambrian, but they must belong to widely separated eras, and must have been pro- duced by entirely different processes. If it is proposed to regard the gneisses as ‘“‘ Archean,” we must refuse to include the Torri- donian strata inthe same section of pre- Cambrian time. But so much uncertainty exists as to the application of this term Archzan, examples are so multiplying wherein what was supposed to be the oldest and truly Archean rock is found to be intrusive in rocks that were taken to be of much younger date, and there are such slender grounds for correlating the so-called Archean rocks of one country with those of another, that I prefer for the present, at least, not to use the term at all. Let me very briefly state some of the main characteristics of the two sharply con- trasted rock-systems of the north-west of Scotland.

The oldest gneiss of that region was originally called ‘“‘ Lew- isian’’ by Murchison, from its large development in the Island of Lewis, and I think it would be, for the present at least, an advan- tage to retain this geographical appellation. At first this ‘fundamental gneiss” was thought to be a comparatively simple formation, and the general impression probably was that it should be regarded as a metamorphic mass, produced mainly from the alterations of very ancient stratified rocks. Its foliation- planes were believed to be those of original deposit which by terrestrial disturbance had been thrown into numerous plications and corru- gated puckerings. But a detailed study of this primeval rock has revealed in it a far more complicated structure. The sup- posed bedding- planes have been ascertained to have nothing to do with sedimentary stratification, and the gneiss has been resolved into a complex series of eruptive rocks, varying from a highly basic to an acid type, and manifestly belonging to differ- ent times of extrusion. With the exception of one district, to which I shall immediately refer, no part of the whole region yet examined has revealed to the rigid scrutiny of my colleagues of the Geological Survey, any trace of rocks which can be regarded as probably of other than igneous origin. It is true that our researches have been hitherto confined to the mainland

DETERS OAV aA), Le OOKES,| 3 Lol DSL A SELES: 5

of Scotland, the large area of the Outer Hebrides, which con- sists of similar gneisses, remaining to be explored. It is therefore possible that indisputable evidence of an ancient sedi- mentary series through which the gneiss was originally protruded, may yet be discovered in the unexplored islands. But taking the gneiss as at present known in Sutherland and Rosshire, we find it to be generally coarse in texture, rudely foliated, and passing sometimes into massive types in which foliation is either faintly developed or entirely absent. Much of this gneiss is considerably more basic than the more typical rocks to which the term gneiss was formerly restricted. It consists of plagioclase felspar with pyroxene, hornblende, and magnetite, sometimes with blue opalescent quartz, and sometimes with black mica. These predominant minerals are segregated in different proportions in the different bands, some bands consisting mainly of pyroxene or hornblende, with little or no plagioclase, others chiefly of plagioclase, with small quantities of the ferro -magne- sian minerals and quartz, others of plagioclase and quartz, others of magnetite. This separation of mineral constituents can hardly be attributed to mere mechanical deformation. It rather resem- bles the segregation layers which may be studied in intrusive sills and other deep-seated masses of eruptive material, and which are obviously due to a process of separation that went on while the igneous magma was still ina liquid or viscous condition. At the same time it is manifest that extensive dynamical changes have affected the rocks since the appearance of this original banded structure.

There is further evidence that beside the original eruptive masses, which for want of any means of discriminating their rela- tive dates of protrusion must in the meantime be regarded as belonging to one eruptive period, other portions of igneous material have been subsequently and at successive epochs, after the first mechanical deformations, injected into the body of the original gneiss. These consist of dykes of basalt and dolerite, followed by still more basic peridotites and picrites, and lastly by emanations from a distinctly acid magma in the form of granites.

6 THE JOURNAL OF GEOLOGY. i:

The oldest or doleritic dykes form a wonderful feature in the gneiss, from their abundance, persistence and uniformity of trend in a west-northwest direction. They have no parallel in British Geology until we reach the crowded dykes of older Tertiary time.

Throughout this remarkable complex of eruptive material, though its different portions present many features that may be compared with those of intrusive bosses and sheets belonging to later geological periods, there is no trace of any superficial volcanic manifestation. No tuffs or agglomerates or slaggy lavas have been detected, such as might serve to indicate the ejection of volcanic materials to the surface. All the phenomena of the Lewisian gneiss point to the consolidation of successively protruded portions of eruptive material at some depth within the crust.

Nevertheless it may yet be possible to show that these deep seated masses have been injected into rocks of older date and of sedimentary origin, and that they have communicated with the surface in true volcanic eruptions. I have already alluded to one limited area where various rocks exist, distinctly different from the prevalent types in the Lewisian gneiss. In the area which is traversed by the long valley of Loch Maree in western Rosshire, there occur clay-slates, fine mica schists, graphitic schists, and saccharoid limestones. These rocks remind us of some of the prevalent members of a series of metamorphosed sediments. The minerals enclosed in the marbles are just such as might be expected in the metamorphic aureole of a granite boss, piercing limestone. But the relations of this group of rocks to the ordinary gneiss of the region are not quite so clear as could be desired, though they seem to point to these rocks being surrounded by and enclosed within the gneiss.

The detailed field-work of the officers of the Geological Sur- vey has made known the remarkable amount of mechanical deformation which the various rock-masses composing the Lewisian gneiss have undergone. These rocks have been com- pressed, crushed, and drawn out, until what were originally mas-

Vd PRECAMBRIAN ROCKS, BRITISH ISLES, 7

sive crystalline protrusions have been converted into perfect schists. The dykes of dolerite have been transformed into horn- blende-schists and the granitic pegmatites have been reduced to a kind of powder which has been rolled out so as to simulate the flow-structure of a lava. There is evidence that most, if not all, of this dynamical change was effected long before the deposition of the Torridonian series, for the latter rests in nearly horizontal sheets, with a strong unconformability upon the crushed and sheared gneiss.

Torridon Sandstone. This group of rocks covers only a limited area in the north-west of Scotland, but it must once have spread over a far more extensive region. It reaches a thickness, as I have said, of 8,000 or 10,000 feet, and consists almost wholly of dull, purplish-red sandstones, often pebbly, and bands of con- glomerate. Dark grey shales, already alluded to as occurring towards the base of the series, are repeated also in the highest visible portion, and have yielded tracks of what seem to have been annelids and casts of nail-like bodies which may have been organic. I have said that the Torridonian deposits which were classed by Murchison as Cambrian, have been proved by the dis- covery of the Olenellus zone in an unconformable position above them, to be of pre-Cambrian age. Except along the line of dis- turbance to which I shall immediately refer, these strata are quite unaltered. Indeed, in general aspect they look as young as the old red sandstones with which Hugh Miller identified them. It is at first hard to believe that such flat undisturbed sandstones are of higher antiquity than the very oldest Paleozoic strata which are so generally plicated and cleaved.

The interval of time between the deposition of the Torridon Sandstone and of the overlying Cambrian formations must have been of enormous duration, for the unconformability is so vio- lent that the lowest Cambrian strata, not only transgressively overspread all the Torridonian horizons, but even lie here and there directly on the old gneiss, the whole of the intervening thick mass of sandstone having been there removed by previous denuda- tion. At Durness, in the north of Sutherland, about 2000 feet of

8 LTE JOURNAL OR NGBOLOGNG

Cambrian (possibly in part Lower Silurian) strata can be traced, the lower portion consisting of quartzites, the central and upper parts of various limestones, sometimes abundantly fossiliferous. Nowhere else in the north of Scotland can so thick a mass of early Paleozoic rocks be seen. Elsewhere the limestones have been in large measure replaced by a complex group of schistose rocks which rest upon the Cambrian strata, and like them dip, generally at gentle angles, towards the east. It was the opinion of Murchison, and was commonly admitted by geologists, that these overlying schists represented a thick group of sediments, which, originally deposited continuously after the limestones, had been subsequently altered into their present condition by regional metamorphism. They were vari- ously named the “ Eastern schists,” the ‘“‘ younger gneiss,” the ‘‘gneissose and quartzose flagstones.” Nicol, who at first shared the general opinion regarding them, afterwards main- tained that they did not belong to a later formation than the limestones, but were really only the old gneiss, brought up again from beneath by enormous dislocations and over-thrusts. We now know from the labors of Professor Lapworth and the officers of the Geological Survey, that Murchison and Nicol had each seized on an essential part of the problem, but that both of them had missed the true solution. Murchison was in error in regarding his younger gneiss as a continuous sequence of altered sedimentary rocks conformably resting on the Cambrian (or to use his terminology, Lower-Silurian) formations. But he sagaciously observed the coincidence of dip and _ strike between the schists and sedimentary rocks below them and inferred that this coincidence, traceable for many leagues, proved that the metamorphism which had given these schists their structure must have taken place after the deposition of the Durness limestones. Nicol, on the other hand, with great insight recognized that there was no continuous sequence above those limestones, but that masses of the old gneiss had been thrust over them by gigantic faults. But he failed to see that no mere faults would account for the coincidence between the structural

TELS PREM A VR KA IN ACOCLGS VERA LESTEL TSIGE,S: 9

lines just referred to in the Cambrian strata, and in the overlying schists, and that the general tectonic structures and lithological characters of the eastern schists differed in many respects from _ those of the Lewisian gneiss.

The problems in tectonic geology presented by the compli- cated structures of the northwest of Scotland have been ably worked out by the officers of the Geological Survey, to whose report in the Quarterly Fournal of the Geological Society for 1888, I would refer for full details. It has been shown that, besides stupendous dislocations and horizontal displace- ments, the rocks have been cut into innumerable slices which have been driven over each other from the eastward, while at the same time there has been such a general shearing of the whole region that for many hundreds of square miles the original rock-structures have been entirely effaced, and have been replaced by new divisional planes, which, when they approach the underlying Cambrian strata, are roughly parallel with the bedding planes of these strata.

In this region, therefore, we have striking proofs of a stupen- dous post-Cambrian regional metamorphism. But there is still much uncertainty regarding the geological age of the rocks which have been affected by it. There can be no doubt that large masses of the old gneiss, torn up from below, have been thrust bodily westward for many miles, and are now seen with their dykes and pegmatites resting on the Durness limestones and quartzites. It is equally certain that in other districts huge slices of the Torridon sandstones have been similarly treated. But where all trace of original structure has disappeared, we have, as yet, no means of definitely determining from what for- mation the present eastern schists have been produced. The ordinary gneissose and quartzose flagstones do not appear to me to be such rocks as could ever be manufactured by any chemi- cal or mechanical process out of the average type of Lewisian gneiss. I have long held the belief that they were originally sediments, but whether they represent altered Torridon Sand- stone, or some clastic formations which may have followed the

10 THE JOURNAL OF GEOLOGY.

Durness limestones, but which have been everywhere and entirely metamorphosed, remains for future discovery. For my present purpose, it is sufficient to observe that, in the meantime, as we can not be sure of the origin of most of the rocks, which, ~ between the West Coast and the line of the Great Glen, have been subjected to a gigantic post-Cambrian regional metamor- phism, it seems safest to exclude them from an enumeration of the pre-Cambrian rocks of Britain.

Dalradian. East of the line of Great Glen, which cuts the Scottish Highlands in two, another group of crystalline schistose rocks is largely developed. It consists mainly of what were undoubtedly originally sedimentary deposits, though they are now found in the form of quartzites, phyllites, graphitic schists, mica-schists, marbles, and various other foliated masses. With them are associated numerous eruptive’ rocks, both acid and basic, sometimes still massive and easily recognizable as intru- sive, sometimes more or less distinctly foliated and passing into different gneisses, hornblende-schists, chloritic-schists, etc. Though it is not always possible in such a series of metamorphic rocks to be certain of any real chronological order of succession, those of the Highland tracts have now been mapped in detail over so wide an area, that we are probably justified in believing that a definite sequence can be established among them. These masses must be many thousand feet thick. Their succession and association of materials are so unlike those of any of the known older Paleozoic rocks of Britain, that they can hardly be the metamorphosed equivalents of any strata which can be recognized in an unaltered condition in these islands. Some traces of annelid casts have been found in the quartzites, but otherwise the whole series has remained entirely barren of organic remains.

What then is the age of this important series? I must con- fess that in the meantime I can give no satisfactory answer to this question. I have proposed, for the sake of distinction and convenient reference, to call these rocks ‘‘ Dalradian.” Murchi- son supposed them to be a continuation of his Durness quartzites,

THE PRECAMBRIAN ROCKS, BRITISH ISLES, II

limestones, and ‘‘ younger gneiss.” His belief may still prove to be in some measure well founded. But at present we have no means of deciding whether the quartzites and limestones of the Central Highlands are the more altered equivalents of the undoubtedly Cambrian strata of the north-west.- It is possible that in the vast mass of metamorphosed rocks constituting the wide stretch of country from the northern headlands of Aber- deen to the south-western promontories of Argyllshire, there may be portions of the old Lewisian gneiss, tracts of highly altered Torridon sandstone, belts of true counterparts of the Cambrian quartzites and limestones of Durness, and, what should not be forgotten, considerable portions of some later sedimentary series which may have followed these limestones, but which, by the great dislocations already referred to, have disappeared from the north-west of Scotland. We are gradually learning more of these rocks, as the detailed mapping of them by the Geological Survey advances, and when the ground on either side of the Great Glen is surveyed, it may be possible to speak with more certainty regarding their true geological relations.

A glance at a geological map of the British Isles will show that the metamorphic rocks of the south-western Highlands of Scotland are prolonged into the north of Ireland, where they spread over a region many hundred square miles in extent. They retain there the same general character and present the same difficult problems as to their true stratigraphical relations. Quite recently, however, a new light seems to have arisen upon these Irish rocks. My colleagues on the Irish Branch of the Geological Survey have detected several detached areas of coarse gneisses, which in many respects resemble parts of the Lewisian gneiss of north-west Scotland. In some cases these areas lie amidst or close to ‘ Dalradian’”’ rocks, but with that obstinacy, which so tries the patience of the field-geologist, they have persistently refused to disclose their true original position with regard to these. Some fault, thrust-plane, tract of boulder- clay or stretch of bog is sure to intervene along the very junc- tion-line where the desired sections might have been looked for.