4i6 
THE TERTIARY VOLCANOES 
BOOK VIII 
occur in little patches, like nearly obliterated ‘ xenoliths,’ points unmistakably 
to a certain amount of absorption ol basic material by the acid magma, with 
consequent enrichment in the ferro-magnesian minerals. 
The slice cuts only the acid rock, which is seen to be of granitoid 
rather than granophyric structure, though the tendency of the felspar to 
enclose quartz-grains is unlike a typical granite. Oligoclase, with combined 
albite- and Carlsbad-twinning, is well represented in addition to orthoclase, 
and some zoned crystals seem to he of albite with a border of oligoclase. 
brown hornblende and a little brown mica are the coloured constituents. 
Magnetite and apatite are also observed.” 
The testimony of the rocks of St. Hilda to the posteriority of the grano- 
phyre to the gabbros and basalts is thus clear and emphatic. It entirely 
confirms my previous observations regarding the order of sequence of these 
rocks in Mull, Hum and bkye. But the St. Hilda sections display, even 
more strikingly than can be usually seen in these islands, the intricate net- 
work of veins which proceed from the granophyre, the shattered condition of 
the basic rocks which these veins penetrate, the remarkable liquidity of the 
acid magma at the time of its intrusion, and the solvent action of this 
magma on the basic fragments which it enveloped. 
o. The Basic Dykes . — Reference has already been made to the numerous 
dykes by which the gabbros of the St. Hilda group of islets is traversed. 
Similar dykes occur also, though less plentifully, in the granophvre. It 
remains lor future observation to determine whether there is one series older 
and another later than the intrusion of the acid rock. In any case, it is 
quite certain that the dykes in the gabbro do not all belong to one period of 
injection, lor frequent examples of intersection may be noticed, especially on 
the cliffs of Borrera, and also cases of double and even treble dykes which 
have been formed by successive infillings within the same fissure. The 
remarkably varied precipices of that island are marked by the long narrow 
ritts left by the weathering of vertical dykes, which, as above remarked, may be 
followed with the eye from the sea-level to the skye-line, ascending obliquely 
across the bedding of the gabbro sheets. Another group of dykes may be 
traced sloping upward at low angles along the face of the cliffs and affording 
admirable ledges with overarching roofs for innumerable gannets, kittywakes 
and guillemots. Other dykes and ribbon-like veins may be seen traversing 
the gabbro in many different directions, precisely as among the Cuillin Hills. 
As no similar network of dykes and veins is to be observed in the granophyre, 
I am disposed to regard a large number of these intrusions as older than 
that rock. But I did not observe any actual example of a basic dyke trun- 
cated by the granophyre. 
There can be no doubt, however, that an injection of similar dykes and 
veins took place after the invasion of the granophyre. These later intru- 
sions are conspicuously displayed along the cliffs that extend from the gabbro 
junction on the north side of St. Hilda round the eastern coast into the 
South Bay. They maintain a general parallelism and ascend from the sea- 
level at varying angles of inclination, running up the pale sea-wall as dark 
