1884.] Professor T. 0. Bonney on the Building of the Alp, 



WEEKLY EVENING MEETING,\/5^ 



Friday, April 4, 1884. 



Sir Frederick Bramwell, F.E.S. Manager and Vice 

 in the Chair. 



Professor T. G. Bonney, D.Sc. F.R.S. Pres. G.S. 



The Building of the Alj)s. 



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, notwithstanding 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 indulgence for 

 leaving some gaps in my reasoning unfilled, and presenting you with 

 little more than an outline. To save time I shall assume a know- 

 ledge 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, Eenevier, 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 prin- 

 ciples which are less familiar, but which I must ask you to bear in 

 mind throughout this lecture : (1) That when a true schist is asserted 

 to be the metamorphosed representative of a post- Archaean rock, the 

 onus prohandi lies with him who makes the assertion ; (2) that rocks 

 composed of the detritus of older rocks may often readily be mistaken 

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

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



