306 Older Palceozoic Bocks, 



By hypothetically placing the Longniynd slates on the parallel of the 

 Skiddaw slates, the successive groups in the preceding tabular sections 

 may readily be made to tally. In papers published during former 

 years, Professor Sedgwick separated the Cambrian series into a lower 

 and an upper division, which were called, respectively. Lower Cam- 

 brian Series and Upper Cambrian Series, and sometimes, after 1836, 

 were called Lower Cambrian System and Upper Cambrian System ; 

 and he adopts the same subdivisions now, with, however, one change 

 of demarcation, for he formerly made his line of separation at the 

 Bala limestone, while he now makes it at the bottom of the lower 

 Bala group of the tabular view. But he never used the word 

 System palceontologically , but only to express a definite sectional 

 group in a great series of deposits ; and he contended many times 

 against the adoption of the words " Silurian System" in any strict 

 palseontological sense, as much as the " System" had neither a good 

 physical nor palseontological base. It appeared to form a part, and 

 a very small part, of the great Cambrian series. That these were 

 his original views, and that he never changed them, he here proved 

 by quotations from the " Proceedings" of the Society, and from 

 other works. During nine or ten years he took for granted (on 

 the unequivocal interpretation of the place of the lower Silurian 

 rocks, as given by Sir R. I. Murchison), that the whole so-called 

 Silurian System was properly determined in its position, and that 

 it rested immediately on the abovementioned upper Cambrian divi- 

 sion of the whole series. But difficulties arose in endeavouring, on 

 this supposition, to join the upper Cambrian groups to the lower Si- 

 lurian. A short historical account is given of these endeavours, 

 which necessarily failed, not because the upper beds of the tabular 

 section had been mistaken by the author, but because the base of the 

 lower Silurian rocks had been misplaced, and their equivalents mis- 

 taken by the author of the Silurian System. The discussion of 

 these questions led to the controversy (Quar. Jour. Geol. Soc, vol. iii., 

 p. 167, 1847) which is here noticed ; and the Professor remarks, that 

 the whole argument of Sir R. I. Murchison is based on a general 

 ideal section, which gives his conceptions of the relations of the Si- 

 lurian System to the other rocks with which it comes in contact. 

 This section he affirms to be not merely imperfect but erroneous 

 in its assumed base, and also wrong in its interpretation of the 

 second of its actual groups. Hence he affirms that the whole 

 argument built upon it comes to the ground. And he further af- 

 firms, that in no part of the Silurian System have the true relations 

 of the Llandeilo flags been made out, either to the beds above them 

 or below them. They are unquestionably the equivalents of the cal- 

 careous slate of Bala, about the relations of which there never was 

 any doubt. Hence he contends that the Llandeilo flags are an 

 upper Cambrian, and not a lower Silurian, group ; and that the 

 Caradoc sandstone is the lowest group made out in a correct section 

 by the author of the Silurian System. Hence, also, ho contends 



