Br. C. Callaicay — Geology of Anglesey. 57 



series,' though I think it likely that field observers may have in- 

 cliuled in it rocks very difl'erent in their real mineral character. 

 But I will assert that the schists on the Anglesey shore, near the 

 Menai Bridge, those in the neighbourhood of the Valley station, and 

 those near the town of Holyhead, thoroughly deserve the title of 

 metamorphic rocks. I believe the idea that these or their equiva- 

 lents are of Bala age has been called ' a working hypothesis.' To 

 myself, who have taken some pains to study rocks, especially of this 

 class, it is about as valuable as one would be that for stratigraphical 

 purposes paid no attention to the species of fossils." 



Prof. Bonney also permits me to quote an extract from another 

 letter, in which, referring to my opinion that in the central area, 

 which he had just examined, the dark-green hornblendic and chloritic 

 schists pass below the granitoidite, he says : — " I must say that the 

 evidence in favour of your view appears to me very strong." 



I did not reply to l)r. Eoberts' paper on Twt Hill (Geol. Mag. 

 May, 1881), because the matter in dispute was one for which I was 

 responsible in a very secondary degree. I should be greatly 

 relieved if he could prove the Cambrian age of the conglomerate, 

 since it contains pebbles of gneiss undistinguishable fi"om the grey 

 variety round Gwalchmai. This fact has always been a stumbling- 

 block to me. If this conglomerate is Cambi'ian, the difficulty is 

 removed, and we are furnished with an additional proof of the Pre- 

 Cambrian age of the gneissic series. 



Dr. Eoberts, in his concluding paragraph, asserts that I have 

 misapprehended some of the most important sections. He then at- 

 tempts to prove this rather formidable proposition by giving one 

 example : the section near Nebo. Passing over this numerical in- 

 consistency, I will content myself with reaffirming my opinion. I 

 have always been perfectly aware that the black shale is faulted 

 against the grit. Indeed, in my note-book, I find a section (dated 

 July 20, 1880) which is almost a facsimile of the figure in Di*. 

 Eoberts' paper.^ My view always has been that the shales were 

 originally unconformably deposited on the grit, and that the latter 

 was afterwards thrust up through the shales, which were thrown off 

 in various directions, but a part remained on the grit, lying in 

 denuded hollows. I freely admit that some eccentric lateral move- 

 ments may have produced the appearances described, and in Anglesey 

 one is surprised at nothing ; but at present I prefer to adhere to the 

 more natural interpretation. 



As I am writing, I may as well refer to some points discussed in 

 Dr. Eoberts' former paper. He attempts to prove that the con- 

 glomerate at Penlon is Cambrian, because, as he asserts, there is an 

 unbi'oken succession down into it from the Cambrian shales. I 

 confess myself puzzled with his mode of working. The exposures 

 are few and distant, especially in the area north of the quarry, where 

 they ai'e most wanted. How one formation can be " traced down " 

 into another on such evidence as this, requires explanation. Anglesey, 



1 Geol. Mag. May, 1881, p. 197. 



