106 The Cambrian and 



the purpose of laying down a principle of common sense and com- 

 mon justice — viz. that if (in the region where the ' overlap' took 

 place) I had blundered in my sections, and mistaken the relations 

 of my upper groups, I was bound to expunge them from my Cam- 

 brian series, and give them up to my friend. On the other hand, 

 if he had misinterpreted the relations of his lower groups (Llan- 

 deilo flags, &c), while I had given their equivalents a right place 

 in my Cambrian sections, he was bound, by the same principle, to 

 give up those lower groups to me. In laying down this principle 

 (in plain prose and not in the language of poetical mockery), I 

 also " appeal to the sense of mankind." 



My friend tells me (Literary Gazette, March 20, and at page 

 355 of vol. Hi. of the Edinburgh Philosophical Journal) "that geo- 

 logists must adhere to his nomenclature, founded on data which 

 have proved to be true." I reply, that my position in this contro- 

 versy is defensive, and not offensive ; that I maintain the nomen- 

 clature first agreed upon ; and that my friend's nomenclature 

 cannot be now adopted, simply because the data on which it was 

 constructed have, out of all question, proved to be untrue. In 

 1834 we visited together (and for the express purpose above men- 

 tioned) what w T e supposed to be the most typical Silurian country 

 both of South and North Wales. And what were the results ? 

 The base line of the Silurian system, to the south of Welsh Pool, 

 was then, I believe, laid down by my friend very nearly as it was 

 afterwards published in his great work. I dictated not a single 

 point of it. And this line was not laid down at random, but on 

 the supposed evidence of sections, as interpreted by himself. 

 Along the whole base line, so far as it was explained to me, the 

 rocks he coloured as Cambrian were supposed by himself to be 

 inferior to his lower Silurian groups ; and that he did not change 

 his views on this material point during the four or five years 

 which followed, is demonstrated by many passages of his great 

 work, among which I may refer to those found in pages 256, 317, 

 319, 343, 356, &c. &c. I then took this line on trust, and there 

 was but one single point of it which we critically examined together. 

 I now know that he misinterpreted his owu transverse sections, 

 and that he made a double mistake — first, in uniting the Llandeilo 

 and Caradoc groups ; secondly, in making them both superior to 

 the rocks which in his great map are coloured as Cambrian. I 

 made no mistake as to these Cambrian rocks ; for on the evidence 

 of my own section I simply affirmed that they Avere superior to 

 the Bala limestone. 



We then examined together the beds on the east side of the 

 Berwyns. My friend pronounced the Meifod beds to be (as they 

 were soon afterwards called) Caradoc sandstone, in its most typical 

 form, ; and from these very bods ho lias derived some of the good 



