﻿Vol. 6 1.] BETWEEN ST. DAVID'S HEAD AND STRCMBLE HEAD. 583 



to note, however, that this mass contains an enstatite-bearing rock, 

 with quartz and micropegmatite, the relation of which to the 

 variolite is not very clear. 



Finally, at Garn Fawr the base of the hill is a coarse-grained 

 doleritic rock with two generations of both felspar and augite, 

 no rhombic pyroxene, and differing from the rock of Garn Gilfach 

 only in the scarcity of magnetite. The summit of the hill is crowned 

 by a tachylytic rock (described by Mr. Cowper Reed), which, 

 from its pronounced columnar structure, seems to be intrusive in 

 the main mass, or, at any rate, subsequent to the coarse rock at the 

 base of Garn Fawr. 



The above-described facts seem to point to the conclusion 

 that this ridge is a composite intrusion. Mr. Cowper Heed 

 explains the occurrence of the granophyric enstatite-bearing rock 

 of Garn Fechan, as due to the incorporation of the acid lava 

 which it penetrates. The analyses tabulated by him 1 show so 

 low a percentage of magnesia in the acid lavas, that it is difficult to 

 understand, upon chemical grounds, how its incorporation could 

 lead to the development of enstatite in a rock which carries no 

 olivine in its normal state. On the other hand, the experimental 

 work of Prof. Vogt has demonstrated that an ordinary non-olivine- 

 bearing diabase-magma might be expected to develop enstatite 

 if enriched by silica and magDesia. 2 I am inclined, therefore, 

 to associate this occurrence with that of the enstatite-diabase of 

 Garn Bolch and Y Garn, and will leave its interpretation to a later 

 stage. The point which I wish to emphasize at present is the 

 occurrence, in this ridge, of rocks differing from the Llanwnda type, 

 and marked by the incoming of a rhombic pyroxene. 



Proceeding next to the St. Nicholas district, a large intrusion is 

 observed on the coast at Llech Dafad. The rock here varies much in 

 grain. The coarsest variety forms irregular streaks and patches in 

 a firier-grained matrix. The coarse rock is paler in colour, owing 

 to a scarcity of ilmenite and other dark minerals, which, as in most 

 of the specimens hereafter to be described, is not confined to the 

 minerals of first consolidation. Enstatite occurs in all the slices to 

 a variable degree, and it always precedes augite in order of crystal- 

 lization. The rock does not differ greatly from that of Garn Bolch, 

 but apatite is more conspicuous. It is an enstatite-diabase. 



At Tresseysilt, close by, is another intrusion of a more variable 

 character. A conspicuous variety recalls the coarse-grained 

 Llanwnda type, but the felspars are more resistant to atmospheric 

 weathering, and stand out in bold relief on the exposed surface. 

 Under the microscope, enstatite can be recognized in all varieties, 

 in some cases as bastite-pseudomorphs, in other cases altered to an 



Juart. Journ. Geol. Soc. vol. li (1895) p. 177. 



Die Silicatschmelzlosungen ' pt. i, pi. i (Ohristiania, 1903). 



