CHAP. XLVII 
THE ACID ROCKS OF ST. KILDA 
415 
mineralogically in the presence of a somewhat larger proportion of ferro- 
magnesian minerals and of soda-lime felspar. These features might indeed he 
matched in many normal granophyres among the Western Isles, but in the 
present case it can hardly be doubted that they are to be explained, at least 
in some degree, by the acid magma having taken up a certain amount of 
material from the basalt. Many of these Tertiary granophyres have., un- 
doubtedly been modified by the incorporation of pieces of basalt and gabbro, 
and a collection made in the Strath district of Skye will furnish examples for 
future study. Professor Sollas’s description of similar phenomena in the 
Carlingford district has already proved the importance of this kind of action. 1 
In the present instance, both brown mica and hornblende occur plentifully 
in the granophyre, and especially round the basalt fragments. This latter 
point is conclusive as to the derivation of the basic material, and further 
proves a certain degree of viscosity in the acid magma at the time of its 
intrusion.” 
Another series of specimens which I collected in the following year was 
submitted to Mr. Harker for petrographieal determination, and his observa- 
tions on two of the microscopic slices are as follow : “ A breccia from the 
South Bay, St. Kilda [7105], consists of angular fragments up to two inches 
in diameter set in a matrix of grey granophyre of medium texture. The 
fragments belong to two types— one of very close texture (basalt), the other 
more evidently crystalline (diabase). Both are cut by the slice. 
“ The basalt shows very evident metamorphism, its augite being wholly 
transformed into greenish-brown hornblende. The little felspar-laths and 
granules of iron-ore seem to be unaltered, though the latter may perhaps 
have contributed to the formation of the hornblende. Another fragment of 
basalt has some larger crystal-grains of augite, and these are not converted 
into hornblende. 
“ The diabase shows a less marked boundary under the microscope, but 
otherwise has similar characters to the preceding. The striated felspar- 
crystals and grains of iron-ore have not been re-crystallized. A considerable 
amount of pale augite remains, but there is also plenty of deeply-coloured 
hornblende, both fibrous and compact. This diabase is certainly an intrusive 
rock, but the basalt, from its petrographic character, might be from a lava- 
flow or from a dyke. 
“The granophyre is of somewhat coarse texture, the micrographic 
structure being only of a rude type. It is notably richer in the darker 
constituents than is usual in such rocks. Further, the hornblende and 
magnetite tend to cluster in little patches which suggest destroyed fragments 
ol basic rocks. A grain or two of sphene occur, a mineral foreign to the 
normal granophyres. 
“Another similar specimen [7106] from the same locality shows a basic 
rock of coarser texture, approaching some of the gabbros in appearance and 
with boundaries in places not very sharply defined. The grey matrix is 
again relatively rich in the dark elements, and the manner in which they 
1 Trans. Roy. Irish Acad. vol. xxx. (1894), pp. 477-572. 
