CHAP. XLVII 
THE ACID ROCKS OF ST. K1LDA 
409 
augite, while the granitoid rock contains a little deep-brown biotite. 
Scattered crystal-grains of magnetite occur in both.” 
Narrow ribbon-like veins of a finer material, sometimes only an inch in 
breadth, traverse the ordinary granophyre. Similar veins run through the 
rock ot the Eed Hills in Skye ; they are sharply defined from the enclosing- 
rock, as if the latter had already solidified before their intrusion. With 
regard to the microscopic structure of some thin slices prepared from these 
veins, Mr. Harker remarks that “ the material of the veins is of a type inter- 
mediate between granophyre and microgranite [6622, 6623]. The chief 
bulk is a finely-granular aggregate of quartz and felspar, the latter very 
turbid ; but in this aggregate are imbedded numerous patches of micro- 
pegmatite, often of perfect and delicate structure. These areas of micropeg- 
matite show some approach to a radiate or rudely spherulitic structure, and, 
in some cases, are clustered round a crystal of felspar or quartz. Some 
granules of magnetite and rare flakes of brown biotite are the only other con- 
stituents of the rock. Although they must be of somewhat later date, there 
is evidently nothing in the petrographical characters of these fine-textured 
veins to separate them widely from the ordinary granophyres of the region.” 
These veins may be compared with the spherulitic dyke that traverses 
the granophyre of Meall Dearg at the head of Glen Sligachan (described at 
p. 381), which, though undoubtedly somewhat younger than the rock that 
contains it, yet presents the very same structures as are visible at the margin 
ot that rock. 1 The material of this dyke and of the' finer veins of St. Hilda 
and the Eed Hills probably belongs to a later period of protrusion from a 
deeper unconsolidated portion of the same acid magma as at first supplied 
the general body of granophyre. 
Undoubtedly the most interesting feature in the granophyre of St. Hilda 
is its junction with the mass of basic rock to the west of it. This junction- 
line runs from about the middle of the chief or south bay (where, however, 
its precise position is concealed under detritus) across the island to the north 
shore, where it descends the face of the precipice and plunges under the sea. 
Important as the actual contact ot the two rocks obviously is in regard to 
their relative date, it has not hitherto been observed or described. Mac- 
culloch noticed “numerous fragments of trap penetrated by veins of 
syenite,’ but he did not see these rocks in place, and, in spite of their 
apparent testimony to the posteriority of the acid intrusions, he was inclined 
to believe that the veins were not real veins, but that the “ trap ” and 
“ syenite ” had a common origin and would be found to pass into each other, 
as he thought also occurred in Mull and Eum. I 11 recent years Mr. Alex- 
ander Eoss, during ids visit to St. Hilda, collected specimens illustrating 
the varieties of gabbro, dolerite and basalt, and showing the intrusion of the 
acid into the basic rocks. As already stated, he was disposed to regard the 
“ granite ” as of younger date than the gabbros, but left the question 
undecided. 2 
1 Quart. Journ. Geol. Soc. vol. 1. (1894), p. 220. 
2 In his paper, Proceed. Inverness Field Chib, vol. iii. (1884), p. 78, Mr. Ross quotes a letter 
