538 EET. J. r. BLAKE ON THE 



formed by local eruptions, we must take the marbled slates and len- 

 ticular pelites of the west, the pelites and halleflintas round Craig-yr- 

 allor, the long band which passes on the west side of Llangefni to 

 Aberffraw Warren, the outlier at Pentreath, the north-western 

 margin of the Eastern District, and the district of the Lleyn. Foi 

 the ordinary sediments of this period we must look to the South- 

 Stack series ; and for the stratified, but volcanically derived material, 

 to the slates and grits of the jS'orthern District, and other special 

 localities. It would be towards the close of this period, but con- 

 tinuous with it, as it in turn is continuous with that of the chloritic 

 schists below, that the remarkable group of agglomerates and dis- 

 turbed masses were formed, such as we find to the north of Cemmaes, 

 at Careg Gwladys and Dinas Llwyd, and perhaps also in Bardsey 

 Island. It would appear to be at a later date than the earliest of 

 these volcanic eruptions, and probably not far from their close, that 

 the acid intrusions and eruptions took place ; though these may have 

 been, and probably were, widely scattered in age. To these belong 

 the granite of Penbryn-yr-Eglwys, the large mass in the Central 

 District, the white granite of the Traeth Dulas district, the whole 

 development east of Parys jyiountain (the corresponding rocks and 

 their associated pyroclasts here reaching the surface), the granite of 

 Twt Hill, and (perhaps at a later part of the period) the quartz- 

 felsites of Dinorwig. 



"We nowhere see here the uppermost rocks which might represent 

 passage-beds to the Cambrian ; nor is it likely we should. The 

 Cambrians and Ordovicians lying on these unconformably, there may 

 be any amount of intervening deposits elsewhere, and such we find 

 at Bray Head. Correlating the Howth rocks with those of the South- 

 Stack series, the succeeding rocks at Bray represent a higher horizon 

 of the same continuous series. 



Founded on these considerations, some of which are very strong, 

 producing a feeling of certainty, and others weaker, leaving a more 

 or less wide margin within which the conclusions are probably 

 correct, I present the following comparative table (p. 539) of all the 

 districts. 



The interval between the lines must not be taken in any way to 

 denote thickness. I have no data worth anything to determine this. 

 If all were sedimentary and fairly continuous, an estimate might be 

 attempted : but what is the thickness of a volcano or of an eruptive 

 mass of granite ? The importance of a group of rocks when all are 

 in the same district and therefore probably treated somewhat alike, 

 may be better measured by their surface. If, however, I were 

 pressed for an estimate of their equivalent value in sedimentary rocks, 

 I should say that 20,000 feet was a very extravagant one for the 

 whole of them except Bray Head, and that 10,000 feet or 12,000 

 feet was probably much nearer the truth. 



EsTABLISHMEJfT OP THE MoKIAIs^ StSTEM. 



Adoption of a J^AiiiE. — This vast and varied series of deposits, 

 which is proved to be Pre-Cambrian in age, cannot remain nameless. 



