RECENT ADDITIONS TO ANCIENT SEDIMENTS. 357 



by those who made no detailed examination of the fossil- 

 iferous strata of the earth's crust. The result of the adop- 

 tion of these views was that the study of stratigraphical 

 geology remained for some time in abeyance, a circum- 

 stance by no means to be regretted, if, as is likely, it was 

 the direct cause of the diversion of geologists' attention to 

 the constitution of rocks, which has resulted in the magni- 

 ficent advances in petrographical science during the last 

 two decades. 



We may at once proceed to treat of various ancient 

 rock-systems, beginning with the lowest, in order to see 

 what advance has been made in our knowledge of them 

 during the last few years. 



1. PRECAMBRIAN ROCKS. 



Although the existence of Precambrian rocks in Britain 

 was long ago satisfactorily demonstrated, it was not until 

 the establishment of the Olenellus zone as marking a defi- 

 nite horizon that our knowledge of these rocks in Britain 

 was placed on a satisfactory standing, and even now 

 acquaintance with these rocks can only be said to have 

 commenced, and a classification of the Precambrian rocks 

 of Britain, and indeed of other parts of the world, is at 

 present premature, for we cannot say how much of the 

 material which enters into their composition was originally 

 igneous, and therefore newer than rock which now lies 

 above it ; chronological classification of these rocks, accord- 

 ingly, though it has been frequently attempted, is of little 

 value. One thing has now been clearly established, namely, 

 the existence of sedimentary" rocks in Britain and elsewhere 

 underlying the Olenellus beds of Cambrian age. Thus, in 

 1890, Sir A. Geikie (2) was able to announce the Precam- 

 brian age of the Torridon sandstone of North-west Scotland, 

 and in the next year he told the Fellows of the Geological 

 Society, in his presidential address (3), that the officers of 

 the Geological Survey of Scotland had obtained proofs of 

 the existence of much sedimentary material in the South- 



