134 ' PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The relation of the peculiar greenish shale of the Amlwch type to 

 these tuffs and breccias is well shown east of Carmers Point. This 

 shale is interleaved with tuff and contains frequent repetitions of 

 finer or coarser volcanic breccia, as well as occasional seams of black 

 shale. 



The breccias south of Carmel's Point, though they are chiefly 

 made up o felsitic detritus, sometimes show a preponderance of 

 fragments of shale. They vary also so rapidly in texture and 

 composition as to suggest that the vent or vents from which their 

 materials were derived must have stood somewhere in the near 

 neighbourhood, if indeed they are not to be recognized in some of 

 the boss-like eminences that rise above the coast. Some magnificent 

 vents, however, have been actually laid open by the waves between 

 Camlyn Bay and Cemmaes. One of these projects in the headland 

 of Mynyddwylfa. It is filled with a coarse agglomerate, among the 

 large blocks in which fragments of quartzite, limestone, felsite, grit, 

 and shale may be noticed. Huge masses of stratified rock, including 

 several beds still adhering to each other, have been torn up and 

 thrown out by the volcano. Some of the blocks of quartzite and 

 limestone are as large as cottages : one conspicuous limestone 

 mass is at least thirty feet long. Dykes of dolerite ramify 

 through the neck and cut across the embedded blocks. The earth- 

 movements which have so powerfully affected the stratified rocks of 

 the district have likewise produced their effects here, superinducing 

 on the agglomerate a rude cleavage in the same general direction as 

 that of the cleavage in the surrounding rocks. 



Another striking vent has been dissected by the sea on the west 

 side of Cemmaes Harbour. It appears to have been drilled through 

 some of the thick limestone bands of the district. Large masses of 

 vertical and crumpled limestone beds, as well as quartzite, have 

 been caught up in the agglomerate, together with abundant blocks 

 of grit, fragments of shale, and pieces of a pale felsite. Numerous 

 narrow dolerite dykes traverse the neck, and where some of these 

 have weathered and left straight clefts with vertical walls of 

 agglomerate or limestone, they present at once a singularly pictu- 

 resque coast-scenery and an impressive picture of the fissures of an 

 ancient volcano. 



There can be no doubt that these volcanic orifices, since they 

 have broken through some of the limestones and quartzites of the 

 Amlwch slates, are younger than the volcanic and other strata that 

 lie to the south-west of them. But they may well have been the 



