66 



NA TURE 



[J lay 15, 1884 



that there is considerable evidence in favour of this break, and 

 some for one between the Pietra Verde group and the stronger 

 gneisses and schists below ; but in mountain regions we fear lu 

 trust our eyes. The evidence, however, in certain districts in 

 favour of a break at the base of the Lustrous Schists is yet 

 stronger. If I am right in regarding the Lustrous Schists as 

 forming one group with the older part of the Bundnerschiefer of 

 the Grisons region, and of the Thonschiefer of Von Hauer in the 

 Eastern Alps, a study of the geological map will show that it is 

 difficult to explain the relation of these beds to the underlying 

 gneisses and schists without such an hypothesis. What I have 

 myself seen in regard to the Lustrous Schists is strongly in favour 

 of a great break in some localities. On the south side of the St. 

 Gothard we have in the Val Piora the Lustrous Schists app at a\ ' 

 in true succession with the representatives of the garnetiferous 

 group of the Val Tremola, yet on the northern side, 111 thi 

 Urserenthal, the latter series is wanting, and the gneisses which 

 underlie it appear to be immediately succeeded by the Lustrous 

 Schists. This, however, might be explained by a complication 

 of faulting and folding. What I have seen in the Binnenthal is 

 harder to explain. At the head of the Hohsand Glacier, just 

 below the peak of the Ofenhorn, we have a coarse but bedded 

 gneiss, which I should correlate with the series immediately 

 overlying the granitoid gneiss so often mentioned as the lowest 

 rock of all. (dancing towards the north, across the snowfield, 

 we see this rock in the base of the Strahlgrat distinctly overlain 

 by the Lustrous series, with its characteristic band of limestone 

 or dolomite. This series swoops down for some 2000 feet, and 

 we cross it in the upper basin of the valley below, while yet 

 further down the valley I detected the characteristic garnetiferous 

 schist, of which, however, there is no great development. If 

 this be the result of faulting and folding only, it is certainly very 

 remarkable. 



But I must linger no longer over details. The pas 

 warns me that I must attempt briefly to describe the general 

 process of the building of this great mountain group of Europe. 

 I have, I hope, proved that the metamorphic rocks of the Alps, 

 if we may trust mineral similarity and mineral and lithologic.il 

 sequence, are vastly older than the Carboniferous pi 

 that in this ancient series a certain succession may be made out. 

 If we may reason from the analogy of other regions, we m; 

 assign to the whole of their latest group (the 1 ustr .us Schists) 

 an antiquity greater than the earliest rocks in which indisputable 

 traces ol organic life have been found. One point, however, I 

 should notice before proceeding further. It might perhaps be 

 said — it has indeed been said — that the crystalline schists and 

 gneisses of the Alps are the result of the great earth mi 

 by which the mountains were upraised, when heat and 1 n - lu 

 changed mud into schists and felspathic sandstone into gneiss. I 

 have shown you that we can trace a mineral succession in tin' 

 crystalline series of the Alpine chain, and that some at least of 

 these are earlier than the Carboniferous period ; but I can add 

 to the proofs that these great rock masses had assumed in the 

 main their present mineral structure when these movements oc- 

 curred. We meet with some rock masses whose structure is 

 doubtless due to the pressure which they have undergone. This 

 is the case with all cleaved rocks, as was lucidly explained 

 twenty-eight years since by Prof. Tyndall in this very room. 

 We meet also with schists, where, from the arrangement of 

 the mineral constituents, we have good reason for supposing 

 that they were developed when the rock mass was exposed to 

 a pressure definite in direction. Here the lines of different 

 minerals, which we believe indicative of an original structure in 

 the rock, are often wrinkled : the more flaky minerals commonly 

 lie with their broader planes parallel, but, notwithstanding this, 

 there is no very definite cleavage in the rock mass, nor tendency 

 to separate easily along the different mineral layers. Spechni us 

 of such rocks may be obtained in the Alps, but there are others 111 

 which the layers have evidently been crumpled up after the period 

 of mineral change : the bands of cpiartz and felspar have been, 

 as it were, crushed together, the flakes of mica are sometimes 

 crumbled and sometimes twisted round into new positions. 



I he subject is a technical one, so I must ask you to accept my 

 statement, without the long details of microscopic work on which 

 it is founded, that the older Alpine rocks frequently testify to 

 having undergone an extraordinary amount of crushing. In the 

 middle of coarse gneisses, for example, streaks and thin bands of 

 a mica schist may be found, which are not due to an original 

 difference of materials, but to the fact that here and there the 

 original rock has yielded to enormous pressure, and has been 



crushed in situ into lenticular bands of rock dust, from which 

 some new mineral developments have taken place. You may 

 notice also in some regions, where you would classify the rocks 

 at first sight as mica schists, that a close examination of the 

 broken surfaces at right angles to what appear to be planes of 

 foliation reveals a structure resembling a coarsish gneiss. The 

 microscope shows that the rock is really a gneiss, somewhat 

 crushed, and that the micaceous layers are of extreme tenuity — 

 mere films, which do not seem to have been original constituents. 

 The gneissic mass has been crushed, cleaved, and on the cleavage 

 planes films of a hydro-mica have been developed. We cannot 

 fail to be struck, when once our eyes have been opened to it, by 

 the frequency of a slabby structure in the more central parts of 

 the Alpine ranges, the surfaces of these slabs being coated with 

 minute scales or films of mica. These are really records of a 

 /age which has been impressed upon the more central 

 and less flexible portions of the Alps during the great earth 

 movements which they have undergone since they were first 

 metamorphosed. 



Thus in the building of the Alps our thoughts are carried very 

 far back in the earth's history, far beyond the earliest strata of 

 the Palaeozoic age. Under what conditions were these great 

 homogeneous granitoid masses of the fundamental gneisses 

 formed? They differ on the one hand from granites, on the 

 other from the ordinary gneisses ; from the former their differ- 

 ences are but slight, and of uncertain value, yet into the latter 

 they appear to graduate. There is nothing like to them in any 

 rock group, and, so far as our knowledge at present 

 goes, they appear to be the records of a period unique in the 

 world's history. This may well be. When the dry land first 

 appeared, when the surface of the earth's trust had not long 

 ceased to glow, when the bulk of the ocean yet floated as a 

 vapour in the heated atmosphere, when many gases now com- 

 bined were free, we can well imagine that the earliest sediments 

 would be deposited under conditions which have never been re- 

 produced. In the later schists, with their more frequent mineral 

 changes, their distinct stratification, and their beds of quartzite 

 and of limestone, we n ty mark the gradual approach to a more 

 normal condition of things. Some, such as the Lustrous Schists, 

 may indeed be contemporaneous with our earliest I 'a In rzoic rocks ; 

 but I confess that to myself the evidence appears more favourable 

 to the idea that all are more ancient than the period which we 

 call Cambrian, and that the majority are so I feel little doubt. 



Supposing, then, that I am right in considering all the Alpine 

 schists, even the Lustrous group, to be pre-Cambrian, we have a 

 vast interval of time which has left no record in those districts of 

 the Alps of which I have been speaking. It is not till we come 

 to the Carboj ' « can identify any pages in the 



life-history of the earth. We are justified with regard 10 these 

 in the following conclusions : — 



That in the place of the Alps there was at that time an upland 

 district, composed of gneisses and schists, in substantially the 

 same mineral condition as they are at present, together with slaty 

 beds in a comparatively unaltered condition, which district was 

 fringed by a lowland covered by a luxuriant vegetation. Prior 

 to tliis time, also, the metamorphic rocks of the Alps had been 

 so far folded and denuded that the coarser gneisses were in many 

 places laid bare, and contributed the materials which we now 

 find in such beds as the Val ( )rsine pudding-stone. Whether 

 there was a pre-Ti iassic mountain chain occupying some parts 

 of the present Alpine region we cannot venture to say, but I 

 think we may unhesitatingly affirm that there were pre-Triassic 

 highlands. 



After the close ol the Carboniferous period, and anterior to 

 the middle part of the Trias, there were volcanic outbursts on a 

 large scale in more than one region of the Alps — notably in the 

 district near and to the east of Botzen. After this commenced 

 a period of subsidence and of continuous deposition of sediment. 

 to have begun earlier and to have been at first more 

 rapid in the eastern than in the western area. Since in the former 

 the Triassic beds are generally much thicker and more calcareous 

 than in the latter, one is tempted to imagine that the eastern 

 area quickly became a coraliferous sea, with an occasional atoll 

 or volcanic island. Henceforward to the later part of the Eocene 

 the record is generally one of subsidence and of deposit of sedi- 

 ment. Pebble beds are rare: the strata are grits, shales (or 

 slates), and limestones. Whence the inorganic constituents of 

 these were derived I cannot at present venture to suggest, but 

 though conglomerates are rare, there are occasional indications 

 that land was not very distant. In the eastern Alps, however, 



