HARD WICKE 'S SC IENCE- G O SSI P. 



81 



THE PRE-CAMBRIAN ROCKS OF ENGLAND 

 AND WALES. 



By W. W. Watts, B.A., F.G.S., &c. 



{Continued from page 59.] 



THE succession of events in this area, while these 

 rocks were being deposited, appears to have 

 been as follows : — 



' 1. Depression of pre-Dimetian land. 



2. Deposit of some sediment with some 

 Dimetian < volcanic action. 



3. Consolidation, metamorphism, and 

 elevation. 



'4. Land areas, with volcanoes, pouring 

 Arvonian { out lava and ashes. 



lian < 



( 5. Metamorphism and Elevation. 



like granite that they were long-considered to be 

 such. 



Arvonian. — The granitoid rocks are flanked to the 

 north-east of Caernarvon by a thick series of felsites, 

 evidently lavas. A similar mass of felsitic lava 

 occurs on the Llanberis lake, showing flow lines or 

 fluxion structure, and containing one band of inter- 

 bedded slate. Halleflintas, breccias, and felsites 

 occur in masses in the Lleyn peninsula, one impor- 

 tant band flanking the Dimetian axis of Rhos 

 Hirwain. 



The Pebidian rocks cover the Arvonians with a 

 certain amount of unconformability, and consist of 

 grits, conglomerates, breccias, agglomerates, and 

 slates. Many of them have a very beautiful ap- 

 pearance. These rocks seem to be thick where the 

 Arvonian lavas are thin, and vice versa — giving some 

 support to Professor Bonney's classification, which 



N.w. 



Clegyn Valley. 



S.E. 



Fig. 59.— Section across St. David's Promontory (Hicks). I, Dimetian ; 2, Pebidian ; 3, Cambrian ; /, fault. 



Trefgarn Rocks. 



N 



^^^^ 



Fig. 60. — Section across Trefgarn, Pembrokeshire (Hicks). 1, Arvonian ; a, Halleflintas, 5, Breccias ; a, Pebidian ; 



3, Cambrian ; f, fault. 



Fig. 61.— Section from Porth Nobla to Aberffraw (Callaway). 1, Halleflinta ; 



5, Dankschist. ■.-, 



3, Quartz-schist ; 3, Limestone ; 4, Grey gneiss, 



(6. Depression of land. 



7. Deposition of materials during a 



gradual submergence. 

 Pebidian f „ _ ? , -,u 



Submarine volcanoes with pretty 



rapid depression. 



Re-elevation and consolidation. 



3. North Wales. — Dr. Hicks extended his conquests 

 to North Wales, but here several other observers 

 have followed and modified some of his results. I 

 may mention Professor Bonney, Mr. Houghton, Pro- 

 fessor Hughes, and Mr. Tawney. The same three 

 systems appear to be represented, but it seems pos- 

 sible that the Arvonian and Pebidian ^re very 

 closely connected here. 



Dimetian. — These rocks occur atTwt Hill, Caernar- 

 von, the Lleyn peninsula, and a few other localities, 

 and are chiefly compact felspathic-granitoidite, so 



considers the lavas and ashes as only the upper and 

 lower parts of the great Pebidian series. Both 

 Dimetian and Pebidian rocks underlie the Cambrian 

 beds unconformably. 



The microscopic aspect of the granitoidite assigns 

 to it a clastic origin, while the felsites are lavas, and 

 the ashes and breccias are for the most part volcanic, 

 and in some instances strikingly like those at St. 

 David's. 



Dr. Hicks has discovered what he considers to be 

 Pebidian rocks in the Harlech Mountains, here again 

 underlying the Harlech or Low Cambrian beds. 



4. Anglesey. — Among the principal workers in this 

 field, I may mention Dr. Hicks, Professor Hughes, 

 Dr. Callaway, Dr. Roberts ; but the geology of the 

 county is so excessively complicated, and compara- 

 tively so little has yet been done, that it is impossible 

 to reconcile the conflicting views. Professor Hughes 



