44 



NA TURE 



[MayS, 1884 



THE BUILDING OF THE ALPS " 



WHEN were the Alps upraised, and what is the age of their 

 building stones ? On the former of these questions there 

 is less diversity of opinion than on the latter ; yet, notwith- 

 standing all that has been written on both, I am not without 

 hope that I may find a few things sufficiently novel to be of 

 interest to a general audience. 



The subject, indeed, is so vast that I must crave your indul- 

 gence for leaving some gaps in my reasoning unfilled, and pre- 

 senting you with little more than an outline. To save time I 

 shall assume a knowledge of the simpler geological terms, asking 

 you only to remember that I always use the word "schist," as 

 I maintain it ought to be used, to denote a more or less fissile 

 rock the constituents of which have undergone so much mineral 

 change that, as a rule, their original nature is almost wholly a 

 matter of conjecture. I must also ask you to remember that, 

 though I have seldom mentioned the names of other workers, I 

 am really doing little more than giving an epitome of the labours 

 of a host of geologists, conspicuous among whom are Heim, 

 Baltzer, Von Hauer, Gastaldi, Lory, Favre, Renevier, and 

 many more, both Continental and English ; I select, however, 

 those facts with which I have myself become familiar during 

 many visits to different districts of the Alps, from the Viso on 

 the south to the Dachstein on the east. 



It is needless, I assume, to explain that mountain chains are 

 the result of lateral thrust rather than of vertical upheaval, and 

 their contours are mainly due to the sculpturing action of heat 

 and frost, rain and rivers, acting upon rocks bent into various 

 positions, and of various degrees of destructibility. There are, 

 however, three principles which are less familiar, but which 1 

 must ask you to bear in mind throughout this lecture : (1) that 

 when a true schist is asserted to be the metamorphosed repre- 

 sentative of a post-Archaean rock, the onus proban li lies with 

 him who makes the assertion; (2) that rocks composed "I the 

 detritus of older rocks may often readily be mistaken for 

 them ; (3) that great caution is needed in applying the principles 

 of lowland stratigraphy to a mountain region. The first of these 

 is, I know, disputed, but there can be little doubt as to its accu- 

 racy ; the second is indisputable, so is the third ; but I will 

 briefly illustrate what I mean by the statement. [Attention was 

 then directed to diagrams of folds and reversals of strata in the 



A1 P S -] . . . , 



The first section to which I invite your attention is in the 

 neighbourhood of the Lake of Lucerne. There are few travel- 

 lers to whom the cliffs of the Rigi are not familiar. Those great 

 walls of rock, along and beneath which the Rigibahn now takes 

 its audacious way, are mainly composed ol enormous masses of 

 conglomerate, an indurated gravel of Miocene age. called the 

 nagelflue. These pebble beds may be traced in greater or less 

 development along the north-western margin of the Swiss Alps ; 

 they attain in the Rigi and the fatal crags of the adjoining Ross- 

 berg a thickness of not less than 2000 feet. The structure and 

 nature of this nagelflue show that it has been deposited by rivers, 

 possibly at their entry into lakes, but more probably, as suggested 

 by my friend Mr. Blanford, on beginning a lowland course at 

 the very gates of the mountains. In this great mass there are 

 indeed pebbles of doubtful derivation ; but we need not hesitate 

 to refer the bulk of them to the mountains which lie towards the 

 east, and we may regard the great pebble beds of the Rigi and 

 the Rossberg as built of the ruins of Miocene Alps by the 

 streams of a Miocene Reuss. Now when we scrutinise the 

 pebbles of this nagelflue we are at once struck by a remarkable 

 fact. The Reuss, at the present day. onl) passes through Meso- 

 zoic rocks when it approaches the neighbourhood of the Lake of 

 Lucerne. It is within the mark to say that quite three-fourths 

 of its drainage area consists of crystalline rocks. Hence schists 

 and gneisses abound among its pebbles, and the same rocks are 

 no less frequent among the erratics which have been deposited 

 by the vanished glaciers of the Great Ice Age on the flanks of 

 the Rigi to a height of 2000 feet above the Lake of Lucerne. 

 Yet, on examining the nagelflue, we find that, while pebbles of 

 grit, and limestone, and chert — specimens of the Alpine Mesozoic 

 rocks — abound, pebbles of schist and gneiss are extremely rare. 

 I had searched for hours before I found a single one. The matrix 

 also of the nagelflue — the mortar which makes this natural con- 

 crete — when examined beneath the microscope, tells the same 

 story. We do not see in it the frequent quartz grains, the occa- 



' Lecture b-; Piof. T. G. Bonney, D.Sc. F.R S.. Pres.G.3., at the Royal 

 Institution, April 4- 



sional pieces of felspar, the mica flakes, which are records of 

 the detrition of gneissic rocks, but it consists of fragments 

 similar to those which form the larger pebbles. It is therefore 

 a legitimate inference that, in this part of the Alps at least, the 

 protective covering of Mesozoic rock in the Miocene age had 

 not generally been stripped away from the crystalline schists of 

 the Upper Reuss, and that since then the mountains may have 

 been diminished and the valleys deepened by at least a mile ver- 

 tically. I have spoken only of the valley of the Reuss, but a 

 little consideration will show that my remarks may be extended 

 to a much larger area of the Oberland Alps. 



I pass now to two other sections : of these the first is in the 

 neighbourhood of Pontresina. Most of the peaks in this region 

 consist of igneous rocks, of gneisses, and of schists, but some of 

 later date are not wanting — as, for example, may be seen in the 

 flanks of the well-known Heuthal. These last are limestones of 

 Triassic age. Here they overlie unconformably a coarse gneiss 

 — in other places they rest on schists presumably of later date ; 

 in fact, the series of Mesozoic rocks of which the above lime- 

 stone is the lowest member — though now to a great extent re- 

 moved by denudation — has clearly once passed transgressively 

 over the whole series of gneisses and schists of the Engadine. 



The second section, or rather group of sections, is some 

 distance away to the south-east, in the region of the Italian 

 Tj rol. Those magnificent crags of the Dolomite mountains, the 

 serrate teeth of the Rosengarten and the Langkofel, the towers 

 of the Cristallo and the Drei Zinnen, the precipitous masses of 

 the Blattkogel and the Marmolata, are built up of rock-, of 

 Triassic age, not of a very different date from the soft red marls 

 which occupy so large an area in the Midlands of England. 

 Follow me for one moment by the mountain road from Predazzo 

 to Primiero. At the former place — classic ground for geologists 

 — we are surrounded by great masses of igneous rocks, the roots, 

 it may be, of long-vanished cones, although we refuse to recog- 

 nise a crater in the valley about Predazzo. As we ascend towards 

 the beautiful Alps of Paneveggio, we pass for a considerable 

 distance over a great mass of red felstone. This belongs to a 

 group of igneous rocks which extend to the westward even 

 beyond the Etsch. It is overlain by the beds of the Trias, 

 commencing with the red Grodner sandstone and passing up 



- 1 into (In- vasl masses of dolomite which form the wild crags 



of the Cimon della Pala and is attendant summits. But as we 

 1 1 on the other side of the piss towards Primiero. we see 

 the Triassic rocks, without the intervention of the felstone, 

 resting upon mica schisK similar to those which occur in many 

 other parts .,f the Alps. Sections of the above kind. \\ 

 needful, might be multiplied indefinitely to prove that between 

 the base of the Trias and the Alpine schists and gneisses there is 

 an enormous break, but we may content ourselves with one 

 other, interesting not only for the completeness of the demon- 

 stration, bill also for the mode in which it illustrates Alpine 

 structure. [Attention was then directed to the section oi the 

 Mont Blanc range as given by Prof. Favre.] 



The Aiguilles Rouges are composed of coarse gneisses and 

 crystalline schists, but on the highest summit there remains a 

 fragmental outlier of stratified and unaltered rock. The upper 

 part of this is certainly Jurassic. Below this comes a repre- 

 sentative of the Trias— much attenuated, as it is generally in this 

 western region, with possibly a remnant of a deposit of Carboni- 

 ferous age. Be that as it may, there is undoubtedly here a great 

 break between the crystalline series and the succeeding Mesozoic 

 or Palaeozoic rock. 



There remains yet one other section to which I wish to direct 

 your attention ; it is near Vernayaz, in the vicinity of the famous 

 gorge of the Trietit. Where the Rhone bends, at Maiiigny, 

 from a south-west to a north-west course, the crystalline massif 

 of the Mont Blanc region of which we have just spoken crosses 

 the river, and is lost to sight as it plunges beneath the Mesozoic 

 rocks of the western summits of the Oberland. The gorge of 

 the Trient is cut through hard and moderately coarse gneiss ; 

 the same rock occurs at the Sallenche waterfall. Between the 

 two is a mass of rock of a totally different character — in part a 

 dark slate, like some in Britain of Lower Silurian age ; in part a 

 conglomerate or breccia in a micaceous matrix, proved by its 

 plant remains to be a member of the Carboniferous series. 

 Omitting some minor details, not without interest, it may suffice 

 to say that we have in this place the end of an almost vertical 

 loop, formed by the folding of beds of Carboniferous age between 

 the crystalline rocks, which are the foundation-stones of the 

 district. The conglomerate is at the base of the Carboniferous 



