64 Professor T. G. Bonney [April 4, 



I know, disputed, but there can be little doubt as to its accuracy, 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 Alps.] 



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

 bourhood of the Lake of Lucerne. There are few travellers to 

 whom the cliffs of the Eigi are not familiar. Those great walls of 

 rock, along and beneath which the Rigibahn now takes its audacious 

 way, are mainly composed of 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 Eigi and the 

 fatal crags of the adjoining Eossberg 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 

 Eigi and the Eossberg as built of the ruins of Miocene Alps by the 

 streams of a Miocene Eeuss. Now, when we scrutinise the pebbles of 

 this nagelflue we are at once struck by a remarkable fact. The 

 Eeuss, at the present day, only passes through mesozoic rocks when 

 it apin'oaches 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 Eigi 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 ex- 

 tremely rare. I had searched for hours before I found a single one. 

 The matrix also of the nagelflue — the mortar which makes this natural 

 concrete — when examined beneath the microscope, tells the same 

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

 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 tlie 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 Eeuss, and that since then the 

 mountains may have been diminished and the valleys deepened by at 

 least a mile vertically. I have spoken only of the valley of the 

 Eeuss, but a little consideration will show that my remarks may be 

 extended to a much larger area of the Oberland Alps. 



