THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE IV. VOL. III. 



No. XI.— NOVEMBER, 1896. 



ORIGHsTAL AETICLES. 



I. — On some Crush-Conglomerates in Anglesey. 1 



By Sir Archibald Geikie, D.Sc, LL.D., F.B.S., 

 Director- General of the Geological Survey. 



rpHE important observations made by Mr. Lam pi ugh. among the 

 JL " crush-conglomerates " of the Isle of Man suggest that the 

 phenomena described by him may have a much wider range than 

 had previously been supposed. Ever since I had the opportunity 

 of going over his Manx evidence with him, I have suspected 

 that some of the fragmental rocks which I myself have regarded 

 as volcanic agglomerates might prove to be due, not to volcanic 

 explosions, but to the same kind of underground movements which 

 have undoubtedly given rise to the enormous masses of "crush- 

 conglomerate" in the Isle of Man. The breccias of Anglesey 

 seemed to me likely on renewed examination to prove to belong 

 to the latter series. Accordingly I recently took occasion to revisit 

 these rocks, both in the centre and along the north coast of the 

 island. The result was entirely confirmatory of my suspicions. 

 The breccias in question are, I now feel convinced, true crush- 

 conglomerates. 



The amount of mechanical deformation which these rocks have 

 undergone is one of their most obvious characteristics. On the 

 supposition of their volcanic origin it was quite conceivable that 

 coarse agglomerates and volcanic breccias might undergo crushing, 

 together with the sedimentary series to which they belonged, so 

 that this evidence of deformation formed in itself no proof that they 

 were not of pyroclastic derivation. But more detailed investigation, 

 in the light of the Manx examples, brings to view proofs that this 

 conglomeratic structure has been produced by the breaking up of 

 stratified rocks in situ. At Llangefni, for example, the strata 

 affected appear to have been originally shales or mudstones (with 

 possibly some fine felsitic tuffs), alternating with bands of hard 

 siliceous grit. They have been crumpled up and crushed into 

 fragments which have been driven past each other along the planes 



1 Eead before the British Association at Liverpool. 



DECADE IV. VOL. III. — NO. XI. 31 



