THE MONIAN SYSTEM OF ROCKS. 463 



32. Oil the MoNiAN System of Eocks. By the Eev. J. F. Blaze, 

 M.A., F.G.S. (Bead March 14, 1888.) 



[Plate XIII.] 



Introduction. 



Is there no order in Pre-Cambrian rocks ? Do they consist in one 

 area of great undulating masses of gneiss, in which those who have 

 studied them refuse to recognize any stratigraphical sequence, and 

 in another of tiny fragments of formations, two or three of which 

 may be contained within an area of a few square miles ? and are 

 these types in no way connected ? Such were the questions which 

 forced themselves upon me after brief consecutive visits to the High- 

 lands and St. David's. Consulting my friend Dr. Callaway on these 

 points, he advised me to go to Anglesey, where I should find rocks 

 that might fairly be compared with the gneiss of the Highlands in 

 character, in close proximity to, if not associated with, volcanic rocks 

 of the type of St. David's. I went ; and after three years' work in 

 summer and spring vacations have arrived at conclusions which I 

 now venture to lay before the Society. 



But in the forefront of this inquiry we are met by the fact that 

 Sir Andrew Eamsay, who knows the district as one knows one's 

 native hills, has declared that there are no Pre-Cambrian rocks in 

 Anglesey at all*. Can we ignore this fact? or must we not first 

 ascertain the reasons which have led to this conclusion ? If after a 

 study of the area, sufficiently prolonged to place one's self somewhat 

 on a level with his knowledge of the rocks, one reads his words, 

 they strike one as so masterly, that if his conclusions are not correct, 

 the correction of them must be undertaken on the spots whence 

 they were drawn, and the cause of error pointed out. 



To prove the existence of Pre-Cambrian rocks in Anglesey it is 

 necessary to establish two points. First, that the rocks we assign 

 to that age are overlain, unconformably if possible, by true Cam- 

 brian strata ; and secondly, that they are so important in develop- 

 ment, and so distinct in character, that we cannot consider them 

 merely lower and metamorphosed portions of the same system, 

 separated by a mere local unconformity f. In the examination of 

 the question, the second of the above points is naturally taken first, 

 since it is necessary to become acquainted with the lower rocks 

 themselves, in order to appreciate any unconformity or overlap of 

 the Cambrian that may exist. And as the examination goes on, the 

 importance of this point becomes so impressed upon the mind, and 

 its proof becomes so thorough, that one is convinced that the first 

 point must be true, whether it can actually be proved or not. But 

 in the demonstration to others it is best to take them in the logical 

 order. 



* Mem. Geol. Survey, vol. iii, 



t See Dr. A. Geikie, Quart. Journ. Geol. Soc. vol. xxxix. p. 291. 



Q. J. G. S. No. 175. 2 i 



