104 "mOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Head to the south have long been recognized as equivalents of 

 some part of the Cambrian system of Wales ; and though Mr. Blake 

 would make them older and place them in his " Monian System"*, 

 I see no good reason for questioning the received opinion on the 

 subject. 



The grits, quartzites, and shales or slates of Howth have, on the 

 whole, an inclination towards the south, so that the lowest parts of 

 the section are to be looked for at the north end. By far the 

 most remarkable members of this group of strata occur towards 

 the base. They consist of beds of breccia, regularly banded with 

 the other strata, on many successive horizons, composed of a grey 

 shaly matrix through which are dispersed in variable quantity 

 angular, subangular, and more rarely rounded pieces of grit, quartzite, 

 and shale, varying in size up to a foot in diameter. The coarsest 

 beds lie towards the bottom, and the included stones become fewer 

 and smaller as the strata are followed in upward sequence until we 

 come to bands with only a single stone stuck here and there in the 

 shaly matrix. The fragments are exactly like pieces of the ordinary 

 strata of the island. Were the occurrence of these breccias an 

 unique phenomenon, one would hesitate to assign an origin to them; 

 but their resemblance to a certain class of volcanic breccias in other 

 Palaeozoic formations leads me to regard them as probably due to 

 contemporary volcanic explosions whereby older portions of the 

 Cambrian deposits of this region were blown out so as to fall in 

 fragments on the sea-floor and be entombed in later accumulations 

 of the same system. 



The evidence obtainable at Bray is less striking, but possibly 

 not less important. The rocks of this promontory, though not 

 precisely like those of Howth, doubtless form part of the same 

 series. I could not find there any trace of the coarse breccias of 

 the more northern locality ; but I observed numerous intercalations 

 of a peculiar dull, pale olive-green or greenish buff-coloured, exceed- 

 ingly fine-grained felsitic material which can be recognized, even 

 from a little distance, by its weathering with a characteristic rough 

 cavernous or carious surface, which contrasts with the smoother 

 joint-faces of the shales and grits. This substance occurs in laminae 

 and bands from less than a line to occasionally as much as a foot in 

 thickness among the shales and grits, but more especially among 

 the former. It resembles some of the fine felsitic tuffs of Bangor and 

 Llauberis and those in the Lower-Silurian series of Wicklow and 

 * Quart. Journ. aeol. Soc. vol. xliv. (1888) p. 534. 



