Rev., J. F. Blake — The Llanheris TJ nconformity . 225 



my words, " eliminating contortions." If I spread out a fan on a 

 table it may be said to lie horizontally upon it, yet all its parts will 

 be highly inclined, and even the horizon itself on a stormy day at 

 sea has not a single level spot upon it. In such a squeezed district 

 as Y Big! it is not by local dips in individual sections, but by the 

 position of corresponding outcrops, that we must judge of the lie of 

 the stratum as a whole. 



The Microscopic Evidence. 



But what does the microscopic evidence amount to ? That some 

 of the minuter fragments (for they require the microscope to see 

 most of them) are derived from the same kind of roclss, whether 

 they be found in Cambrian or Post-Llanberis strata. These rocks 

 are for the most part of a common kind, with no particular character 

 to render identity certain, but we will grant they are the same. At 

 that early epoch there was not much choice of materials to derive 

 a fragment from, and any of them must have come either from 

 Cambrian rocks themselves or from the Pre-Cambrian of the 

 neighbourhood. In the wearing down of the Cambrian strata to 

 make the conglomerates, the elements of the coarser rocks in them 

 would yield smaller elements to the succeeding rocks, and these 

 would be found in the matrix. But the character of the later 

 rock would be shown by the larger fragments it contains. The 

 coarser Post-Llanberis rocks ai'e always thus distinguishable. The 

 rederivation of some parts of the conglomerates is interestingly 

 shown at Moel Tryfaen, where one of the pebbles is itself 

 conglomeratic. 



It is not the fragments that are identical in the two series that 

 present any difficulty, but rather the origin of those larger stones 

 whose home we can find neither in Cambrian nor Pre-Cambrian 

 rocks, but which closely resemble rocks of Post-Llanberis age. 

 Some of these perhaps might be matched approximately in Anglesey, 

 but they seem too large to have come so far. I think it not at all an 

 unreasonable hope that we may some day find a fossiliferous pebble. 

 I much regret that my opponents in this matter should have taken up 

 the other side of the question, for, if they had sought to do so, there is 

 no one more likely to be able to tell us whence the lai'ge pebbles may 

 have come. In such an investigation the microscope would be doing its 

 proper work ; but it cannot prove by the similarity of small fragments 

 the continuity of the deposits containing them, any more than it can 

 disprove by the difference of the fragments the identity of two 

 conglomerates, as has been already shown in the case of Twt Hill 

 and Careg Goch. The worker with the microscope must not forget 

 that in geology he is the servant of the stratigraphist, for until he 

 knows the conditions of occurrence of his rocks his account of them 

 has no geological value. It is therefore quite beyond his province to 

 attempt to prove microscopically such a stratigraphical conclusion as 

 the non-existence of an unconformity. The attempt in the present 

 instance is founded on the assumption that rocks formed when " the 

 same rocks were undergoing denudation " must belong to the same 



DECADE IV. VOL. V. — NO. Y. 15 



