106 J. W. DAWSON — SOME RECENT DISCUSSIONS IN GEOLOGY. 



address as president of the geological section of the British Association, 

 insisted on the unity of origin of the older crystalline rocks with their 

 more modern successors, and the veteran Prestwich * has made a strong 

 protest against an exaggerated uniformitarianism as applied to the later 

 formations. Here also we need to beware of that one-sidedness which 

 has led to so much unnecessary controversy from the days of Werner 

 and Hutton down to the present time. 



We may be fully prepared to admit that, on the hypothesis of a cooling 

 globe, there must have been certain primitive rocks deposited as the first 

 products of the action of a heated ocean on a still hot crust, conditions 

 which would not again occur except in limited and exceptional cases. 

 On the other hand, we know that ever since land and water existed, there 

 must have been a certain uniformity and continuity of erosion and 

 deposition. We may also in all this expect a kind of development 

 whereby old rocks are wasting away and are redeposited in somewhat 

 different states, but we must at the same time make allowance for the 

 differences provided by alternate elevation and subsidence and by the 

 occasional introduction of igneous products. So guarded, we may hold 

 with truth that there has been a substantial uniformity of the origin and 

 character of rocks throughout geologic time, though in every succeeding 

 age the continents and the rocks composing them are different from their 

 condition in any previous period. There has thus been uniformity with 

 change and progress, but while the laws of nature and the operations 

 under them have been uniform in kind, we must beware of supposing 

 that they have been uniform in rate. In short, slow and gradual actions 

 inevitably produce catastrophes or critical periods, and these again pre- 

 pare the way for the recurrence of times of dull uniformity and scarcely 

 perceptible motion. Slow and secular accumulation of sediments on 

 limited areas or expansion and contraction of rocks may produce sudden 

 and violent movement of the crust, just as we have seen lately the 

 accumulation one by one of sheets of paper at length involve in sudden 

 and utter ruin a great public building. A cliff long acted on by disinte- 

 grating atmospheric agencies at length falls instantly in a mass of frag- 

 ments, and this prepares the way for new action of the atmosphere on 

 the cliff in its protracted and infinitesimal way, and for the agency of the 

 waters in removing the talus of fallen material. 



The stupendous changes which we know our continents have expe- 

 rienced in the later Cenozoic periods and in times comparatively short, 

 should warn us against exaggerated uniformitarianism, more especially 

 when we find that this opposes invincible difficulties in the way of any 

 rational explanation of such climatic changes as those of the Glacial 



* Nineteenth Century, October, 1893, 



