THE HIGHLAND BORDER ROCKS OF THE ABERFOYLE DISTRICT. 
181 
The spilites are green, compact, non-porphyritic rocks, occasionally vesicular, and 
sometimes veined with yellow epidote. Pillow structure has not been observed, 
owing perhaps to the unsatisfactory character of the exposures, but the usual 
association with chert has been noted. The rocks are sometimes shattered and 
broken, and the specimens from Gualann exhibit frequently a well-marked flaser 
structure. 
In thin section all the spilites are seen to be highly altered, and in the majority 
of the specimens examined the characters of the original minerals are quite unde- 
terminable. The outlines, however, of some of the constituents (notably the felspars) 
are still discernible, and the igneous texture is clear. The felspar of the freshest 
specimens is albite ; the ferro-magnesian minerals are completely decomposed, and 
are represented by secondary chlorite and carbonates. Many of the rocks consist 
entirely of secondary minerals — chlorite, quartz, carbonates, colourless micas, epidote, 
and iron oxides. An exceptionally fresh spilite from Gualann exhibits good variolitic 
structure (Plate III, fig. l) ; other examples show good fluidal arrangement of felspar 
laths ; only rarely are the rocks micro-porphyritic, with small phenocrysts of felspar. 
Amygdales of the vesicular varieties consist of quartz and chlorite ; they are small, 
and usually almost circular in section. 
In their mineral composition and texture and in their association with siliceous 
sediments the spilites resemble the green basic lavas associated with the Cambrian 
and Ordovician formations elsewhere in Britain. Spilites have already been described 
from Arran # in the west and from the Forfarshire-Kincardineshire f area in the 
extreme east, and their recognition in the Aberfoyle district adds another link to the 
chain of lithological evidence which has been used to correlate the various separated 
areas that make up the belt of Highland Border Rocks. 
The Cherts and Shales . — The spilites are overlaid by a group of compact, fine- 
grained, fossiliferous sediments consisting of grey cherts and black carbonaceous 
shales, usually intimately associated, grey cherty shales, and mudstones. Every 
gradation is found between hard almost pure cherts on the one hand, and soft mud- 
stones and shales on the other. The cherts are often finely banded, with alternating 
darker and lighter coloured laminae, the thickness of which frequently shows a rapid 
variation even within the limits of a hand specimen, but is usually \ inch or less. 
Breccias of angular chert fragments set in a matrix of black shale are of common 
occurrence along crush lines. The cherty shales often show an imperfect cleavage 
rudely parallel with the bedding planes, and this feature is particularly well marked 
in the belt of cherty shales adjoining the Leny Grits. The sediments of this series 
are often traversed by minute veinlets of quartz, chalcedony, pyrites, and carbonates. 
In thin section the cherts are seen to consist almost entirely of chalcedonic silica 
in a finely granular condition, throughout which is irregularly distributed opaque 
* Mem. Geol. Survey : The Geology of North Arran , etc., pp. 18-21, 1903. 
f Trans. Roy. Soc. Edin., vol. xlviii, p. 926, 1913. 
