PRE-CAMBRIAN ROCKS OF ST. BAVIB's. 323 



important additions to our knowledge of metamorphism may be 

 looked for from a further study of the rocks of St. David's. 



5. The Diabase Dykes ajs t b Intrusive Sheets. 



The latest rocks of the St. David's district are the dykes and in- 

 trusive sheets of diabase, which have been referred to in Part I. as 

 traversing all the other formations. The dykes are specially abund- 

 ant in the granite. One or two may be detected in almost every 

 artificial opening which has been made in that rock ; while on the 

 coast- section they are here and there profusely abundant. They 

 are likewise frequent in the quartz porphyries, as may be seen in 

 the quarries near the schools, and still more conspicuously on the 

 cliff south of Nun's Chapel, where, at a picturesque sea-worn cave, 

 four dykes, varying from one to nine feet broad, cut through the 

 elvan. They occur also in the volcanic group and in the sandstones 

 and shales above the conglomerate, but become fewer in number as 

 they recede from the granite centre. 



In external characters, the rock composing these dykes and sheets 

 may be described as usually a dull dirty-green or yellowish brown 

 mass, to which the old name of " wacke " might appropriately be 

 given. It exhibits the texture and mode of weathering of the more 

 distinctly crystalline members of the basalt family. It is occasion- 

 ally amygdaloidal or cellular, the kernels or cavities being arranged 

 parallel with the sides of the dyke. Here and there a rudely pris- 

 matic structure extends between the walls. 



To the descriptions of the microscopic structure of this rock 

 already given by Prof. Judd, Mr. Davies, and Mr. Tawney I have 

 but little to add. It is a diabase, but more allied in structure to 

 true basalt than the olivine diabase of the volcanic group. It 

 especially differs from the older rock in the abundance and freshness 

 of its felspars, in the comparative scarcity of its augite, and in the 

 absence of olivine. The magnesian silicates are very generally 

 replaced by green decomposition-products diffused through the mass. 

 An occasional crystal of hornblende, recognizable by its cleavages 

 and dichroism, may be detected. 



I may add that some of these diabase dykes present excellent 

 examples of fluxion-structure. Mr. Tawney noticed this in one of 

 those traversing the granite at Porth-lisky*. A thin slice which I 

 have had cut from probably the same dyke, and showing the contact 

 of the rock with the granite, repeals the streaming of the plagioclase 

 prisms along the wall of the dyke. I have found the same arrange- 

 ment in the narrow dyke that cuts through the shales south of 

 Castcll. But the most beautiful example among my slides was taken 

 from a dyke in the shales, in a small cove to the east of Nun's Chapel. 

 The shale and eruptive rock are in contact ; and the small acicular 

 prisms of felspar, besides ranging themselves in lines parallel to the 

 side of the dyke, stream round the larger felspar crystals (Plate X. 

 fig. 12). 



* Proc. Nat. Hist. Soc. Bristol, vol. ii. pt. 2, p. 115 (1879). 



