Vol. 65.] ORDOVICIAN AND SILURIAN ROCKS OF CONWAY. 171 



The most recent references appear to be those in Mr. Harker's 

 work on the ' Bala Volcanic Series of Caernarvonshire ' (1889). 

 He seems to be the first to have drawn attention to the fact that 

 the rocks lying west of the Penmaenbach Fault are not all intru- 

 sive, as the 1-inch Geological Survey map would seem to indicate, 

 but similar in many respects to those found on Conway Mountain 

 itself. 



II. Succession op the Beds. 



In all these papers no hint is given as to the structure of the 

 country, and up to 1896 the existence of the unconformity between 

 the Ordovician and the Silurian rocks had been generally accepted as 

 a certainty. The discovery of Upper Bala and Upper Llandovery 

 Beds seemed to indicate, however, that this unconformity was at 

 any rate not so great as had been previously supposed, and it 

 seemed advisable that the relationship between the two rock-series 

 should be re-examined and mapped over a wider area. This has 

 now been done, with the result that continuously lower Llandovery 

 Beds have been found ; and it has become obvious that in the 

 Conway district, at any rate, the unconformity does not exist. 

 There is, on the contrary, a perfectly unbroken succession from 

 the Volcanic Series to the Denbighshire Grits and Flags : also, as 

 might therefore be expected, the newer beds share every degree 

 of the folding and cleavage of the older rocks, and are indeed at 

 times even more intensely folded. 



Detailed mapping has moreover brought to light some interesting 

 facts, which affect the age of the Volcanic Series hitherto regarded 

 as belonging to the Bala. Everywhere succeeding the Ashy Beds, 

 which are the highest members of the Volcanic Series, there 

 is found a belt of black shales containing a rich graptolite- 

 fauna. The fauna of the highest shales is closely comparable 

 with that of the Lower Hartfell Shales of the South of Scotland; 

 but the remaining and greater proportion of the beds contain 

 a characteristic assemblage of Upper Glenkiln graptolites, and must 

 therefore be of Llandeilian age ; moreover, the Volcanic Series 

 includes within it a band of shale containing Lower Llandeilo 

 graptolites. 



This would seem to indicate that the main Volcanic Series, so far 

 as the Conway district is concerned, must be relegated to the 

 Llandeilian. The view held by the Geological Survey palaeonto- 

 logists in 1881 is correct as regards the age of the Black Shales, 

 although the idea that the shales were out of their proper place in 

 the succession does not seem to be justified. 



The Conway succession appears to be as follows : — 



