328 The Geological Bearing of Mr. Davison s Paper. [May o, 



nental denudation are chiefly deposited. Hence, the continents grow 

 by the formation of mountain- chains along their borders. 



3. The rate of mountain-making, and therefore also that of conti- 

 nental evolution, diminishes with the increase of the time. 



IV. " Note on the Geological Bearing of Mr. Davison's Paper." 

 By T. G. Bonnet, D.Sc., LL.D., F.R.S., Professor of 

 Geology in University College, London. Received April 7, 

 1887. 



The results obtained by Mr. Davison throw light upon one or two 

 matters in regard to the petrology of the older rocks, which have 

 always appeared to me difficult of explanation. I venture therefore 

 to add a brief note to his paper, written from the point of view of a 

 geologist. He throws light especially on the following matters : — 



(1.) Among the older rocks the great foldings and their results, 

 such as cleavage, appear to have occurred when the beds formed the 

 upper layers of the earth's crust. Thus the Ordovician rocks of 

 North Wales were cleaved anterior to the deposit of the Silurian ; 

 the Carboniferous, and other Palaeozoic rocks of South-west 

 Britain and Britanny were plicated and cleaved, geologically 

 speaking, shortly after their deposition. The great foldings in 

 the Scotch Highlands occurred, in great part at least, in Silurian 

 time. The disturbance of the Lake District rocks, resulting in 

 cleavage, must be placed between the end of the Silurian and the 

 very beginning of the Carboniferous ; that of Southern Scotland, 

 between perhaps yet narrower limits. The first epoch of mountain 

 making in the Central Alps, with its plication and cleavage, imme- 

 diately followed the deposition of the Eocene rocks. The list might 

 easily be extended. 



(2 .) The crystalline substratum often appears to be less modified than 

 : the overlying softer and more recent beds. This I had attributed to 

 the greater resistency of the former, but then could not see how to 

 explain the foldings of the latter, if the others were comparatively 

 uncompressed. This, however, accords with Mr. Davison's results of 

 the diminishing effects of compression, while the fact that in early 

 geological times the " neutral zone " between compression and tension 

 was comparatively near the surface of the earth, may explain the 

 frequent parallel arrangement of the minerals in the older Archaean 

 gneisses. I do not now refer to the more marked changes, such as 

 the intercalation of calcareous or micaceous rocks, of more felspathio 

 -or quartzose layers, whereby a stratification is simulated, if it be not 

 recorded, but to the fact that very often a general parallelism may be 



