542 EEY. J. F. BLAKE ON THE 



the bursting forth of siliceous or calcareous springs, which have 

 produced sporadic limestones or quartz-rocks. The volcanic erup- 

 tions seem to have been most localized towards the close. Thus the 

 lower portions are formed from the fine dust of some large eruption ; 

 the higher show the centres at various fixed spots. To this group 

 properlj' belongs the title Pebidian ; but so far as the intrusive rocl^ 

 associated with volcanic products can be assigned to the same age 

 as their hosts, it must include the so-called Dimetian also; and 

 some of the lava-flows of the period have been called Arvonian. If 

 the name Pebidian is to be retained, it must be for this only, or, 

 since it corresponds to the rocks which alone occur at St. David's, 

 it might be called the St. David's Group. It is probable that to 

 this group we must assign the fossiliferous ashes of Berth Wen, 

 in which case it possesses characteristic fossils, one of which is 

 Ortliis Bailyana. The other type, which is related to the first 

 somewhat in the same way that the Caradoc sandstone is to the 

 Bala ashes, is limited to the island of Holj^head, on the south 

 side of the great fault. It has been called in this memoir the 

 South- Stack Seeies. A small amount of volcanic debris may 

 have contributed to this series at Gogarth, but otherwise it is well 

 stratified and bedded. It is also contorted on the large scale and 

 greatly cleaved, and foliation is developed along the cleavage and 

 not along the bedding-planes. It is also characterized by masses of 

 white quartzite, which, in some cases, are certainly bedded, but in 

 others are of obscure origin. On the whole it is a slaty series, and 

 contains soft bands, while many of the beds are comparatively thin, 

 and have to be measured in inches rather than in feet. This series 

 extends across the Irish Channel to Howth Head, where it exhibits 

 similar characters, the slaty portions being to the north, and the 

 more ashy to the south. No fossils have yet been discovered in this 

 group. These two types are believed to be contemporaneous, because 

 the chloritoid schists are continuous with both, one on the one side 

 and the other on the other. Moreover there is a certain resemblance 

 between some portions of them, as when we compare the northern 

 slaty district with the South-Stack Series. These types therefore 

 form the Middle Mois^an". 



The third grou]D is not found in Anglesey or at St. David's, in 

 both which places the basal Cambrians rest on no higher rocks than 

 the middle or volcanic group ; but on the other side of the Channel 

 the well-known rocks of Bray Head succeed those of Howth, and 

 are yet of a difi'erent character. They are massive grits and slates, 

 some strongly cleaved, but most are uncleaved. The beds are thick ; 

 there are great quartzite-masses, and chlorite is still developed. 

 These form the Beat-Head Geoup. The fossils which have been 

 found in them become characteristic fossils of the Uppee Moj^ian. 



Develop3iekt oe Mokean" Bocks is othee Areas. 



The principal area in England which, it is probable, consists of 

 Monian rocks is the Longmynd. The slates and grits of this area 

 are unconformable beneath the Stiper Stones, and they are not seen 



