CAMIiKIAN SHALES NEAR CAERNARVON. 



137 



deposited, and that the volcanoes were sub-marine. The presence 

 of fine sediment over so many parts in Britain, and in other 

 regions in Western Europe proves also that there was no land of 

 any extent near. 



The immediately preceding and the succeeding sediments also arc 

 for the most part such only as would be formed in a moderately deep 

 sea and away from shore. The fine argillaceous material, however, 

 was undoubtedly the result of denudation, though in this case of dis- 

 tant land. Up to this time the sediments had been gradually 

 changing from the rougher to the finer materials; and in the next 

 group, wherever volcanic action was not taking place over these 

 areas, the rocks heaped up were formed almost entirely by marine 

 life, showing the presence then of a clear sea. Exception might 

 perhaps be taken to this view on the evidence of the great grit bed 

 in parts of Caernarvonshire, and also of the Stiper Stones in Shrop- 

 shire. It must not be forgotten, however, that in each of these 

 places very fine sediments directly overlie and underlie these quart- 

 zose sandstones, and quite conformably. My own opinion, already 

 expressed in a former paper *, is that the grit bed and the Stiper 

 Stones are parts of what was a shifting sandbank at a considerable 

 distance from land, and in a sea of moderate depth. 



The discovery of these fossils by Mr. Marr must be looked upon 

 as of considerable importance, since it enables us to read the succes- 

 sion of these early Palceozoic rocks in Caernarvonshire in a much 

 clearer manner than we have hitherto done, and it also carries the 

 succession there a stage higher. 



The following attempt at a comparison of the scries which make 

 up the Arenig group in Caernarvonshire and in Pembrokeshire 

 may now be useful. 



Pembrokeshire. 

 ( a, Black shale followed 

 by iuterbcddecl tuff. 



Upper Arenig. * 



Middle Arenig. 

 Lower Arenig. 



Caernarvonshire. 

 Shales of Bangor 



and 

 Caernarvon, and shales 

 under and mixed up 

 with the lower ash-beds 

 in the Arenig moun- 

 tains, and to the N.E. 

 and N.W. of Portma- 

 doc. 



b, Fine slates b, Slates at Ty Obrey , and 



under the above near 

 Arenig. 



Slates and flags Grit bed at Ty Obrey &c. 



Slates and flags Upper Tremadoc rocks 



near Portmadoc, and 

 black slates under the 

 grit bed near Arenig &e. t 



* Quart. Journ. Geol. Soc. vol. xxxi. page 175. 



t In addition to the evidence given in my former paper in regard to these 

 beds, I have lately received f rom Mr. Homfray a good specimen of Callograptus, 

 which was discovered by Mr. Ash in the neighbourhood of Portmadoc, in the 

 lower part of the Upper Tremadoc rocks, and which appears to be identical 

 with the species found, at St. Davids. 



