ON THE CAMBRIAN ROCKS IN N.W. CAERNARVONSniRE. 271 



21. 0)1 the Camerian and associated Kocks in jS'orth-west Caer- 

 narvonshire. J3y Prof. J. F. Blake, M.A., F.G.S. (Read 

 December 21, 1S87.) 



Introduction. 



The area to which the present memoir refers has for a long time 

 attracted the attentiou of geologists, partly because within it may 

 be found the oldest portion of the Cambrian series, and therefore the 

 supposed base of our known stratified rocks, and partly because, of 

 late years, there has been an expectation of finding here some repre- 

 sentatives of still older series. 



The most recent observers, indeed, agree in regarding the whole 

 of the mass coloured as porphyry between Bangor and Caernarvon, 

 and some part of that coloured altered Cambrian, as belonging to a 

 Pre-Cambrian epoch, and the t'elsite of Llyn Padarn andMoel Tiyfaeu 

 they treat in the same way. There is no difference of opinion 

 between them on this latter area, but as to the lormer, though all 

 divide the mass into three parts — the mass near Caernarvon, the 

 mass near Dinorwic, and that near Bangor, — Prof. Hughes considers 

 all three a connected series of " beds," Prof. Bonney regards the 

 Caernarvon rock as very old, and the other two as connected, while 

 Dr. Hicks regards them as representing three independent and uncon- 

 formable groups. Again, Prof. Bonney and Prof. Hughes diff'er as 

 to the stratigraphy near Bangor, with the effect that the former 

 considers much more to be Pre-Cambrian than the latter does. ALL 

 three views are diametrically opposed to that of the Survey, not only 

 as to theories but as to many of the facts. Nor has the Survey 

 given way. Prof. Bamsay, in 1881*, says the proposed changes 

 aie made on purely theoretical grounds; and Prof. A. Geikie, in 

 1883 1, certainly accepts no change of view, on the ground that, if 

 the Survey maps are to be corrected, it must be done in the same 

 style as that in which they w^ere constructed. 



It will be probably admitted therefore that there is need for 

 further observation and proof one way or the other. 



I have been led personally to the study of this area from its rela- 

 tion to the rocks of Anglesey. If the schists of that island are 

 really Pre-Cambrian, this fact must be proved, if possible, by tracing 

 the base of the Cambrian, or of some older rock, across from the 

 mainland into contact with them. 



It will be well at the outset to state clearly the position to which 

 this study has led me. I find that, though there may be Pre- 

 Cambrian rocks in this area (and I am inclined, from considerations 

 derived from other areas, to think there are), yet the evidence on the 

 ground itself is not sufiieient to justify this conclusion. In other 

 words, the views of the Survey, exception being made of certain 

 " convictions " with regard to metaniorphism, most clearly approxi- 



* Mem. Geol. Surv. vol. iii., new edition, 

 t Quart. Journ. Greol. Soc. vol. xsxix. 



