﻿276 MR. J. V. ELSDEN ON THE [May 1908, 



Enstatite, if it was ever present, is now represented by pale 

 irregular patches of chloritic matter. There are also some partly- 

 digested fragments of country-rock and some quartz-xenocrysts. 

 The latter have corroded outlines, fringed with a shell of small 

 augite-crystals, precisely similar to those noticed by Mr, Harker in 

 the marginal part of the Skye gabbro (see PI. XXX, fig. 1). These 

 are evidently undigested remnants of the country-rock : they occur 

 in all my slides from the contact-zone. A series of slides was 

 prepared from this zone beneath the south-eastern flank of Carn 

 Llidi, in order to ascertain, if possible, to what extent assimilation 

 had taken place. Quartz-xenocrysts can thus be traced to a distance 

 of about 6 feet from the margin, beyond which distance rhombic 

 pyroxene begins to be well individualized, and, except in texture, 

 the rock mineralogically resembles that of the main mass. 



An interesting point in connexion with these marginal rocks is 

 the occasional appearance of a delicate micropegmatite within 10 

 yards of the margin, where the grain of the rock is still fine, owing 

 to rapid cooling. This occurrence of micropegmatite so near the 

 margin, its general distribution throughout the more acid portions 

 of these intrusions, and its absence from the aplite-veins, are 

 significant in connexion with theories respecting the origin of 

 granophyric structure.^ 



On the whole, the chief marginal modifications which these 

 intrusions exhibit seem to be the result of rapid cooling, combined 

 with a certain amount of assimilation of country-rock. 



Structural modifications. — There are no true porphyritic 

 crystals in the marginal zones, from which it may be inferred that 

 the magma was intruded in a wholly liquid state. In a general 

 sense the texture of the rock becomes coarser in proportion to the 

 distance from the cooling surfaces ; but there are cases where coarse 

 and fine-grained rocks are in close juxtaposition. The rocks vary 

 in texture from subophitic to granular, and are occasionally almost 

 pan-idiomorphic (see PI. XXX, fig. 5). 



A distinct banded structure is exhibited in parts of the St. 

 David's-Head mass. This is particularly evident in the neighbour- 

 hood of the cromlech known as Coetan Arthur, north-west of 

 Perth Melgan. The bands consist of thin alternate layers of 

 light and dark rock, often not more than an inch wide. On 

 the weathered surface the light bands stand out in relief, and 

 contrast strongly with darker and more decomposed layers between 

 them. The strike of the foliation is approximately parallel to that 

 of the intrusion. Thin sections of the light and dark bands show 

 that they differ mainly in the relative proportions of their con- 

 stituent minerals. In the light bands felspars and quartz pre- 

 dominate, while in the dark bands pyroxenes are more abundant. 

 The distinction agrees generally with that displayed by the basic 

 and acid types of rock described above. The minerals in the banded 



^ See R. H. Rastall, * The Buttermere & Ennerdale Granophyre ' Quart. 

 Journ. Geol. Soc. vol. Ixii (1906) p. 270. 



