﻿Vol. 6 I .] ANNIVERSARY ADDRESS OP THE PRESIDENT. lxix 



and in the South- West of Ireland, shows the strictly-local value of 

 the test in the case of these Lower Palaeozoic rocks. 



I have elsewhere commented upon the comparative absence of 

 volcanic rocks from the sediments of Jurassic and Cretaceous ages, 

 which is in so marked a contrast with their abundance in Palaeozoic 

 and Tertiary times, suggesting a period of comparative quiescence 

 from volcanic activity in the later Mesozoic Era. How far this 

 apparent absence is due to our want of knowledge, remains to be 

 seen ; but, considering the extensive development of known Jurassic 

 and Cretaceous rocks over so many different parts of the world, I 

 cannot help thinking that the rarity of observed cases of volcanic 

 ejectamenta among these rocks indicates a period of lull. 



(4) Variation in the Nature of the Earth-Movements. 



It is unnecessary to comment upon the influence of earth- 

 movement in affecting the lithological characters of the sediments. 

 The alternation of movements of a positive and negative character 

 has been utilized as assisting in the classification of sedimentary 

 rocks, but the applicability of this to areas of wide extent is a 

 matter upon which some difference of opinion still exists. A study 

 of the strata over those parts of the world which have been investi- 

 gated in fullest detail, vaguely suggests the predominance of positive 

 movements over very wide areas at the end of pre-Cambrian times, 

 and later during the Devonian, Permo-Triassic, and Miocene 

 Periods, and of negative movements in the intervals between these 

 periods. But it is doubtful how far this suggestion is due to our 

 imperfect knowledge. The influence of the Scandinavian move- 

 ments over regions where geology has been most assiduously studied 

 may, for instance, give us an exaggerated notion of the importance 

 of positive movements in Devonian times ; and even in Europe we 

 find, in passing southward, a continuous development of marine 

 sediments connecting those of Silurian with those of Carboniferous 

 times. These may have been laid down in an ocean extending far 

 to the south, for when we next meet with deposits of Devonian 

 age, namely in South Africa, they are of marine origin. 



At one time, the existence of physical breaks in the succession 

 of the strata, owing to the occurrence of positive movements, was 

 very largely utilized for classificatory purposes, many of the major 

 divisions or systems of the sedimentary rocks having been defined 

 owing to the existence of unconformities separating the rocks of 



