44' PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [ Juile 3, 



I could make myself acquainted with, I rejoice in such discoveries, 

 because they si ill better link the two groups together in one indis- 

 soluble natural system. At the same time, referring my readers to 

 the introductory chapters of the work on Russia, and requesting them 

 also to consult the tabular lists of the Upper and Lower Silurian fos- 

 sils of Scandinavia, I beg them to remark, that whilst some typical 

 species of those regions are omitted, or have not yet been detected in 

 England, they have been found on the same geological horizon in North 

 America. It is from a combination of all these facts, that although 

 when I published the ' Silurian System ' I thought and hoped that 

 there might be found a different group of animal life in the western 

 and northern regions of Wales and in those tracts occupied by the 

 so-called " Cambrian rocks," which, without critical examination 

 on my own part, I left to be decided by the researches of Professor 

 Sedgwick, I now repeat that an appeal both to those tracts as well as 

 to other regions of the globe since I addressed the Society to that 

 effect from the Chair, has sustained the conclusion, that the fossili- 

 ferous portion of the system alluded to in my work as Cambrian, is 

 at length shown to be inseparable from the Lower Silurian type ; 

 and that whilst this group passes stratigraphically and zoologically 

 into the Upper Silurian, the latter is characterized by many typical 

 and peculiar fossils. 



Admitting that " organic changes are the surest guides in making 

 out the history of successive changes on the surface of the globe," 

 Professor Sedgwick says that " they form a part only of our evidence, 

 and that the great physical groups of deposits, however rude Or 

 mechanical, are historical monuments of perhaps equal importance 

 in obtaining any true and intelligible record of past events*." Now, 

 if by this it is meant, that without duly noting all the synchronous 

 and posterior eruptions by which the surface of the earth has been 

 modified, as well as the density of marine sediments in one region 

 and their tenuity in another, no geological description of a given 

 region can be complete, every one must subscribe to the position. 

 But, on the other hand, I maintain, that if thick accumulations re- 

 plete with igneous rocks and dislocations, do not contain as many 

 relics of former inhabitants as strata of much less vertical dimen- 

 sions (occupying the same geological horizon), the latter ought to 

 be appealed to in preference to the former, as superior evidence for 

 compiling the history of the earth. 



Professor Edward Forbes has clearly shown, that what he has 

 laid open in existing nature cannot but have been true in the re- 

 motest antiquity, and that in former as well as in the present seas, 

 animal life can only have existed down through a given thick- 

 ness. Now, assuming that the Lower Silurian is the oldest type 

 of animal life, we might expect that at that period there would 

 be a diminution in the numbers of the animals in proportion as 

 the strata were accumulated at greater and greater depths, and 

 that in those protozoic days there might have been depths of the sea 



* Quart. Geol. Journ. Vol. ii. p. 129. 



