PKE-CAJOKIAN KOCKS OP ST. DAVID'S. 273 



have been introduced. Most of these are greenish in colour and 

 more or less earthy in texture. Where the opposite walls have 

 been displaced, slickensided surfaces may be seen upon them and 

 upon the substances interposed between them. But these appear- 

 ances present no features differing from what are universally found 

 among massive jointed rocks. They cannot be confounded with 

 any original structure of the mass. 



Occasionally veins of a paler colour and finer texture ramify 

 through the rock. They vary in width from an inch or less up to 

 more than a foot. Every one familiar with a large mass of granite 

 will recoguize such veins at once as characteristic features of it. 

 These also are referred to more in detail in Part II. 



The only distinct species of rock which we could discover in the 

 mass is a dull-greenish, more or less decomposing, diabase or wacke, 

 occurring in the form of abundant dykes and veins. These vary 

 from a few inches to several feet in breadth, and traverse the 

 granite irregularly in all directions. Where several run parallel at 

 a short distance from each other, and have a slight hade in the 

 same direction, they produce a deceptive resemblance to an alter- 

 nation of beds. 



That these dark-green rocks, however, are all eruptive, intrusive, 

 and of later date than the granite, may be confidently inferred on the 

 following grounds : — 1st. They have precisely the ordinary external 

 forms of eruptive dykes and veins, ramifying in different direc- 

 tions, coalescing and reuniting. 2nd. They present the usual micro- 

 scopic characters of dykes of diabase or ancient basalt-rocks, with 

 which they agree in crystalline structure, in the presence and linear 

 arrangement of amygdules, in the existence of a rude prismatic 

 structure transverse to the walls, and in their tendency to sphe- 

 roidal weathering. 3rd. In microscopic structure they unmistakably 

 belong to the b'asalt family. 4th. They not only traverse the 

 granite (being most abundant in it) but are found cutting through 

 the Cambrian beds on many different horizons. The true character 

 of these eruptive rocks appears to have been first detected by 

 Prof. Judd *, — a conclusion confirmed by Mr. Tawney f. Dr. Hicks 

 draws a distinction between some of them which he admits to be 

 intrusive and others which he seems disposed to regard as bedded 

 in the " Dimetian " mass J. But for this distinction I was unable 

 to discover any ground whatever. There can be no question as to 

 their universally intrusive character and late Cambrian or Post- 

 Cambrian date. 



In some of his earlier papers Dr. Hicks refers to the occurrence 

 of abundant shales among the more crystalline rocks of the ridge. 

 In his more recent summaries no reference is made to such inter- 



* Quart. Journ. Geol. Soc. vol. xxxiii. p. 235 (1877), and xxxiv. p. 156 (1878). 



t Proc. Brist. Nat. Soc, new ser. vol. ii. pt. 2, pp. 113 et seq. 



\ Quart. Joum. Geol. Soc. vol. xxxiv. p. 156. In his more recent papers he 

 omits mention of these rocks as integral parts of his " Dimetian " series, unless 

 they are included in the "hornblende and chloritic schist," which he states to 

 be also a portion of the same series. 



