39 ° 
THE TERTIARY VOLCANOES 
BOOK VIII 
granophyre appears liere at the surface, the alteration of the basalts is 
probably to be attributed not so much to the influence of the gabbro, as to 
the abundant acid sills, dykes and veins, for there may be a considerable 
body of granophyre underneath the locality, the dykes and veins being 
indications of its vicinity. 
In the summer of 1895 I examined the locality with much care, and 
collected some typical specimens illustrative of the conditions of meta- 
morphism presented by different varieties of the bedded basalts. Thin 
slices cut from these specimens were placed in Mr. Harker’s hands for 
microscopical examination, and he furnished me with the following notes 
regarding them. 
“In hand-specimens the bedded basalts from the neighbourhood of the 
gabbro of Loch Scavaig [6613-GG1 8] do not appear very different from the 
normal basalts of this region. The most conspicuous secondary mineral is 
yellowish-green epidote in patches, and especially in the amygdales. 
“ The texture of the rocks varies, and the slices show that the micro- 
structure also varies, the augite occurring sometimes in small ophitic plates, 
sometimes in small rounded granules. The chief secondary change in the 
body of the rock is shown by the augite, which is seen in various stages of 
conversion to greenish fibrous hornblende. Some round patches seem also 
to consist mainly of the latter mineral, and are probably pseudomorphs after 
olivine. Here the little fibres are confusedly matted together, without the 
parallelism proper to uralite derived from augite. No fresh olivine has been 
observed. The felspar and magnetite of the basalts show little or no sign 
of metamorphic processes, unless a rather unusual degree of clearness in the 
felspar crystals is to be regarded in that light. 
“ The contents of the metamorphosed amygdales are not always the 
same. Epidote is usually present in some abundance, and in well-shaped 
crystals. It has a pale citron tint in the slices, with marked pleochroism ; 
but a given crystal is not always uniform in its optical characters. Fre- 
quently the interior is pale, and has a quite low birefringence. This is 
probably to be regarded as an intergrowtli of zoisite in the epidote, and 
there are a. few distinct crystals of zoisite seen in some places. 
“In the slide which best exhibits these features [G613] the crystals of 
epidote are in part enwrapped and enclosed by what are doubtless zeolitic 
minerals. At least two of these are to be distinguished. One, very nearly 
isotropic, and with a pale-brownish tint, is probably analcime. Associated 
with this is a colourless mineral with partial radiate arrangement and with 
twin lamellation ; the birefringence is somewhat higher than that of quartz, 
and the 7 -axis of optic elasticity makes a small angle with the twin-line. 
These characters agree with those of epistilbite. In other parts of the same 
large amygdale, the epidote crystals are embedded in what seems to be a 
felspar. This latter mineral is rather obscure, and twin-lamellation is rarely 
to be detected ; but it seems highly probable that felspar has here been 
developed by metamorphic agency at the expense of zeolites which once 
occupied the amygdale. I have observed undoubted examples of this m 
