300 Geological Society : Prof. Sedgwick on the 



been multiplied, laws respecting the distribution of organic types 

 were discovered, which not merely superseded, in many large for- 

 mations, all classifications founded on mineral structure ; but often, 

 through wide regions, gave indications of succession which were 

 unsupported by the direct evidence of sections. As, however, the 

 (so called) laws respecting the distribution of organic types, are 

 mere general results grounded on actual observation, it is obvious 

 that they can never upset conclusions drawn from the clear and un- 

 ambiguous evidence of sections. The two methods may be used 

 independently, and conspire to the same end ; but in their nature 

 cannot come into permanent collision. 



The author then points out some examples in which these principles 

 had been violated. (1) The attempt formerly made by some geolo- 

 gists to arrange the Stonesfield slate in a tertiary group, merely from 

 the presence of certain fossils of a class not commonly found in second- 

 ary rocks. (2) Some of the doctrines put forth in the papers of M. 

 Deshayes, which if pushed to their utmost extent would make the 

 evidence of sections of no value ; whereas without sections fossils 

 could never have led to any general laws of succession. (3) The 

 recent discussions respecting the age of the culm plants of North 

 Devon. The plants were assumed to be of the age of the greywacke, 

 from the mineral structure of the rocks in which they were imbedded ; 

 or the rocks were assumed to be of the carboniferous period by the 

 species of the imbedded plants : whereas true geological reasoning 

 required that, anterior to either of the preceding conclusions, the 

 true position of the culm measures should be determined by actual 

 sections. 



The author then goes on to point out the difficulty of classify- 

 ing the vast series of schistose rocks below the old red sandstone 

 — from the great resemblance of their mineral type — from the 

 absence of well-defined beds of organic remains in many large re- 

 gions — and from their entire disappearance in the last members 

 of the descending series. The Silurian system is almost the only 

 exception to this remark ; and even this system is developed 

 in many parts of England without any distinct succession of natural 

 groups. The mineral type is on the whole much more uniform in the 

 great series under notice than in the secondary system of England ; 

 but the frequent absence of organic remains, and of any succession 

 of distinct groups, is compensated by the enormous scale of deve- 

 lopment, as shown in the natural sections : and the author concludes, 

 that it is not by hypothetical views and analogies, or by maintaining 

 one part of geological evidence at the expense of another ; but by 

 applying every kind of evidence in its proper place, and above all by 

 actual surveys and detailed sections, that we can ever hope to bring 

 into coordination the complicated phaenomena of which he is only 

 attempting to give a brief synopsis. 



TWO CLASSES OF OLD STRATIFIED ROCKS, &C. 



The author first notices the older stratified series of Scotland, and 

 divides it into two classes. 



