114 THE OLDER ROCKS AT ST. DAVIDS. 



little strings of green rock, not vertical, but intruded along the 

 joints of the massive Dimetian ; they are so thin that they have 

 become quite decomposed, and run dou^n to a green sandy earth ; 

 they would probably have passed unnoticed if unmistakeable dykes 

 had not been seen in their vicinity. A microscopic slice was taken 

 from the i^-foot vertical dyke for comparison with the other cases 

 of supposed chloritic schists or indurated shales which are noticed 

 below 3 its structure has been already described in the memoir 

 above cited. 



On the coast at Forth Liskey a dyke has been noticed by 

 Dr. Hicks {I. c. p. 235) which is vertical, and shows horizontal 

 columnar structure. Close by are thick masses of green rock, 

 which are described by their discoverer as chloritic schists, 

 effervescing with acids, and containing some of them over twenty 

 per cent, of carbonates. It appeared, however, to Mr. Huddleston 

 and to Professor Hughes, our companions, that these were better 

 to be interpreted also as dykes. Certainly the appearance in the 

 field seemed to show that this was the more likely explanation as 

 they break about irregularly, at one time horizontal, at another 

 spot arching up towards the before-mentioned vertical dyke, with 

 which they are probably united either above or below — these 

 portions show no columns. For microscopic study, slices were 

 taken from both the vertical columnar dyke and the thick inclined 

 mass of green rock. The structure was found identical, and 

 tallying also with that of the Forth Clais dyke. The Calcite is in 

 crystals, polysynthetic, and evidently of secondary origin. As the 

 felspars have been decomposed so the calcite has been deposited. 

 The bisilicate has been entirely replaced by Viridite, not even any 

 crystal form having been preserved. Opacite in cubical masses 

 and grains is abundant. 



For the preliminary notice as above cited. Dr. Hicks has, I 

 believe, substituted in a forthcoming paper another in which their 

 igneous origin is assumed. He speaks above {I. c p. 234) as if 

 the vertical dvkes " had been intruded into the series before this 



