312 The Rev. Prof. Blake — Base of the Sedimentary Series. 



overlying conglomerates, and found that the latter "was made out 

 of the fragments of the former. In the same way Prof. Hughes, 

 near Llanerchymedd, and Mr. Eoberts at Penlon and Nebo, found 

 conglomerates at the base of the so-called " Silurian." From the 

 running of the line of junction on the Survey map, and the con- 

 spicuous character of the conglomerates along this line, it is obvious 

 that these facts were perfectly familiar to Prof. Eamsay. Why then 

 did he not, like Dr. Hicks and Prof. Hughes, consider them to prove 

 the Precambrian age of the granite and associated rocks ? The 

 answer is obvious. He mapped the conglomerates not as Cambrian 

 but as Silurian ; and the evidence on which he did this is equally 

 plain. The only fossils ever obtained in the rocks which lie con- 

 formably above the conglomerates are such as need not be older 

 than the Arenig. It is true that at Llanfaelog there is a good 

 quantity of grit between the fossils and the conglomerate ; but this 

 is not the case elsewhere, and we do not know whether there are 

 any strike faults here or not. It is therefore an entirely ungrounded 

 assumption that the conglomerates are basal Cambrian, in the sense 

 that they underlie the diminished representatives of Menevian, Lingula 

 Flags, and Llanberis slates. Dr. Hicks has therefore in no way proved 

 that the rocks are Precambrian, or brought the slightest evidence 

 against the conclusions of Prof. Eamsay. The first real evidence of 

 this kind is that supplied by Dr. Callaway. By his description of the 

 rocks and demonstration of their great complexity and thickness, he 

 rendered it obvious that so distinct and vast a group could not be 

 a mere downward extension of any other group, but must be worthy 

 of a place apart. If any place apart is assigned to it, this must be 

 below the Cambrian. 



But unless we can show an unconformity between these and the 

 true base of the Cambrian, the separation remains a matter of 

 appreciation only. Fortunately the very spots where Prof. Eamsay 

 read continuity, by being better exposed now prove unconformity. 

 Thus on the east of the island near Beaumaris, Prof. Bonney has 

 shown that, the lower rocks, which are continuous across the island, 

 and of the same general character as the rest, are either overlaid by, 

 or are faulted against, a peculiar group of stratified rocks largely 

 composed of volcanic materials, and by tracing it further I have 

 shown that the junction is a real unconformity. Now these upper 

 rocks are continued into the mainland, and there we learn by 

 stratigraphy that they cannot possibly be younger than basal 

 Cambi'ian in the most extreme sense of the word. From whence 

 it follows that the underlying rocks are really Precambrian. It 

 may be taken therefore as settled that the main mass of the schistose 

 slaty and associated rocks of Anglesey belong to a period which 

 antedates the Cambrian. This proved, three questions still remain, is 

 this true of all of them ? do they form one system or several '? and 

 can any part of them be correlated with the rocks of St. Davids ? 



In answer to the first question, Prof. Hughes has pointed out 

 that in the Northern district the beds have a uniform dip to the 

 north, and near the summit of the series, as thus ascertained, there 



