156 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 25, 



I made no mistake in my general interpretation of the Cambrian 

 series, upper as well as lower. To join the Cambrian series to the 

 Silurian was physically impossible, because a great error had been 

 committed at the point where the upper end of the one and the lower 

 end of the other ought to tally. By whom had this error been com- 

 mitted ? Not by myself. But I did all honour to the author of the 

 * Silurian System.' For twelve years, during which I never revisited 

 his typical country, I believed his base-line to be unassailable ; not 

 because I had examined it critically, but because it was he who had 

 laid it down. Tvnce (in 1834 and in 1843) I changed the nomen- 

 clature of some of my upper groups, to bring them into a supposed 

 accordance with his Silurian types, and each time I was driven from 

 my hypothesis by a downright reductio ad absurdum ; and I after- 

 wards returned to my first nomenclature, because I found my sec- 

 tions consistent and true in principle, however imperfect some of them 

 might have been in finish, and in the exhibition of minute details. 



This historical statement was absolutely necessary to my present 

 purpose ; for all I have published on the questions discussed in this 

 paper has appeared, I might almost say, in a fragmentary form in 

 our Proceedings and Journal. Without this statement it might seem 

 that there had been no steadiness or consistency in my views. But I 

 have been so far consistent, that I never shifted a single group below 

 the Bala limestone. And as to my upper groups, though I twice 

 shifted their place, hypothetically, in the hope of bringing them into 

 more near coincidence with the Lower Silurian groups, yet each hypo- 

 thetical adjustment was abandoned after trial ; and I returned to my 

 first grouping and nomenclature because my original sections were 

 right, and because the Silurian sections, at their base, were not merely 

 imperfect, but positively erroneous. 



§ 5. General conclusion. 



It is plain that the author of the ^ Silurian System ' had gradually 

 lost his confidence in his own base-line ; for, in a short sentence of 

 his great work (p. 308), he tells us of the possibility of being in- 

 duced, at some future time, to move his Silurian base to some greater 

 depth * ; yet in the next page he tells us that Moel-ben-tyrch is un- 

 doubted Cambrian, although it is superior to the Bala limestone. 

 But questions might have been asked, which, if I mistake not, ought 

 then to have been answered in the affirmative. Would not a change 

 of the base-line necessarily imply some change of nomenclature ? — 



* In 1834, my friend, on the evidence of the sections, unequivocally excluded 

 the Bala limestone from his lower Silurian rocks, although this limestone was 

 filled with well-preserved lower Silurian fossils. Assuming the truth of the Silu- 

 rian sections, this evidence was perfect demonstration; for the Llandeilo flags 

 were in the Silurian sections placed above all the undulating slate-rocks of South 

 Wales, while the Bala limestone was obviously below a considerable portion of 

 them. That all the older rocks of the Cambrian series were to be called Silurian, 

 provided they contained certain Silurian species, was, therefore, an after-thought 

 with which I had no means of becoming acquainted ; and I believe that this after- 

 thought could never have been seriously entertained, had he not discovered that 

 he had mistaken the sectional place of his Llandeilo group. 



