1862.] HTJLL CAEBONIFEEOTTS STRATA. 145 



ment*. The whole series, therefore, may be considered as a system 

 of wedges lying with their thin edges pointing towards the escarp- 

 ment of the Chalk ; and the absence of these formations under the 

 Cretaceous Eocks at Harwich (for an account of which we are indebted 

 to Mr. Prestwich) is, I submit, a proof of the soundness of the views 

 here adyancedf. 



Is it not therefore a remarkable circumstance, that the north has 

 been the source for the supply of so many non- calcareous formations, 

 including those of the Carboniferous, Triassic, Liassic, and Oolitic 

 Periods, and that there has been a general " northern drift," re- 

 peated at intervals from a period so far remote (at least as far as the 

 commencement of the Carboniferous) until that immediately pre- 

 ceding our own epoch ? Such a series of events, when we consider 

 the great physical changes which have occurred throughout this 

 enormously long period, must, I think, be traced to some general law 

 regulating the course of oceanic currents, and exhibits a remark- 

 able uniformity in the operations of nature through long periods of 

 geological history. 



The extent of the land which was capable of supplying so vast a 

 quantity of material must have been very large, and, judging by the 

 characters of some of the Carboniferous and more recent strata, seems 

 to have been composed principally of granitoid or metamorphic 

 rocks. Its southern limits may have reached the western and northern 

 coasts of Scotland ; and the Highland mountains may have formed 

 outlying islets and headlands. 



§ III. Summary of Conclusions. 

 (General.) 



1. It appears, from the above considerations and examples, which 

 further research will enable us to multiply, that calcareous strata are 

 distinct from argillaceo-arenaceous, not only from differences of ori- 

 gin (a fact now generally admitted), but also in the manner of their 

 distribution ; so that limestones ought to be removed from the class 

 of rocks termed " sedimentary." 



2. That in any natural group or system of strata, consisting, on 

 the one hand, of " sedimentary " strata, and on the other of calca- 

 reous, it appears that the direction of the greatest vertical develop- 

 ment of the one will be that of the smallest vertical development of 

 the other. In a word, where the one becomes thin, the other becomes 

 thick. 



3. That, on the principles here stated, the frequent occurrence of 

 natural groups of rocks consisting of three members, the first and 

 third '' sedimentary," the second (central) calcareous, admits of ex- 

 planation. 



* " On the South-easterly Attenuation of the Lower Secondary Eocks, &c.," 

 Quart. Joum. Geol. Soc. vol. xvi. 



t In my work on 'The Coal-fields of G-reat Britain,' I have given a full 

 exposition of these views, and a section showing the limits of the Carboniferous 

 and Mesozoic Eocks over the South-east of England (pp. 253 etseq., 2nd edit.). 



VOL. XVni. PAET I. L 



