EEV. J. F. BLAKE Olf THE 



them. It is here, then, that the whole difference lies. Given this 

 starting-point, and the consequences which would naturally follow, 

 however remarkable, might be loj-ally accepted. Whether the 

 clearer section now exposed in Baron-Hill Park would have modified 

 his views I cannot say. We may suppose from his words that he 

 worked this district from the east, and by the gradual changes ob- 

 served in descending the rocks of Bangor he was prepared for a 

 further change into schist, and this change would be subsequently 

 discounted in all his researches to the west. Had he commenced 

 on the west instead, and, wearied with the monotony of the schists, 

 come suddenly on these grits and halleflintas, he would have been 

 more struck by the change, and would perhaps have inquired more 

 closely whether they were conformable or not. Nor could he have 

 assigned the differences to metamorphism without a much freer 

 belief in its power to change the constitution of a rock than the 

 teachings of the microscope will now permit. 



DeSCEIPTION of the PeE-CaMBEIAN EoCKS of AlfGLESET. 

 IXTEODTJCTION-. 



The second point to be proved is the importance and distinctness 

 of the series of rocks wluch thus underlie the Cambrian. From 

 such a point of view they require a fuller and more pictorial 

 description than if they were merely altered forms of beds well 

 known elsewhere. 



It may be thought, however, that they have been adequately de- 

 scribed already, and certainly there are two descriptions extant, one 

 from a Cambrian, the other from a Pre-Cambrian standpoint. The 

 first of these, contained in vol. iii. of the Memoirs of the Geological 

 Survey, is chiefly devoted to the illustration and explanation of the 

 contortions and metamorphism to which the rocks have been sub- 

 jected ; so that, full as it is of beautiful touches which recall vividly to 

 the reader the facts he has observed in the field, it is quite inadequate 

 as an account of an important system of rocks. The second, by 

 Dr. Callaway, published in the Quarterly Journal of this Society, 

 vol. xxxvii., approximates much more closely in its general cha- 

 racter to what is required, and were it possible to assent to his 

 statements, this memoir might never have been written, at all 

 events in its present form. But my interpretation of the facts 

 observable differs so widely from his, and the resulting order of 

 succession, both vertically and horizontally, requires so radical a 

 change, that the only feasible plan is to begin the description again 

 de novo, referring only to his descriptions and those of Sir A. Bamsay 

 when they can be accepted so completely as to require no change. 



The Pre-Cambrian rocks of Anglesey, supposing all that are about 

 to be described are accepted as such, are divided, partly by upheaval, 

 and partly by faulting, into no less than six distinct areas. Pour 

 of these are coloured as altered Cambrian on the Survey Map, and 

 two as altered Silurian. These four may be called respectively the 



