THE OLCER ROCKS AT ST. DAVIDS. 11^ 



exceptional. The bulk of the cliff section shows felspar to the 

 unaided eye, but it is apparently more translucent and less opaque 

 than at St. Davids, the quartz may be more abundant in proportion 

 and the chlorite less than at Forth Clais, St. Davids, &:c. In the 

 thin section which I had made for the microscope both Orthoclase 

 and Plagioclase are present. The Quartz contains liquid cavities* 

 Microscopically the appearance of the rock is noted by Dr. Hicks 

 as emphatically different in this higher portion of the series from 

 what it is lower down : this harmonises with the idea of the 

 metamorphic origin. The rock has much more tendency here to 

 schistosity or obscure lamination than elsewhere, its fracture under 

 the hammer being markedly different here to what it is nearer 

 St. Davids city -, it splits into flaggy and rhomboidal pieces owing 

 to concealed laminae coated with a chloritic thin liningr — along 

 these planes the rock parts. The bedding planes are well marked 

 while elsewhere it is difficult to say which divisional planes are 

 dominant, or less irregular than the others. 



The Dimetian immediately N.W. of the city may next claim our 

 attention : we are here close to the fault which runs down the 

 valley bringing Pebidion against the Dimetian, with as Dr. Hicks 

 has shown, a difference of 90*^ in the strike of the two formations. 



The quarry below the Board-school is nearest the fault, and 

 shows us the lowest beds of the Dimetian. The quarry is disu sed 

 and matters are not as clear as one would like 3 it seems to me 

 however, that we have there three distinct rocks; (r), at the left 

 a fine grained greenish and blue-grey rock, which is probably 

 Dimetian in a somewhat altered condition j (2), a dyke of green 

 basic igneous rock, nearly vertical in position apparently and 

 much decomposed -, it is only seen at the left of the excavation 

 forming the face for a small area only, behind it is (3), a quartz 

 porphyry, this I state to be an irruptive rock ; the section in its 

 present state does not show the relations of the three rocks as we 

 could wish, but from various considerations I interpret them as 

 above, viz., the Dimetian altered by the intrusion of the quartz- 



