3io 
THE TERTIARY VOLCANOES 
BOOK VIII 
peculiar colour and pleochroism already noted, and the felspar is of the same 
kind as before.” 
I did not succeed in finding in place any bands of dunite, but this basic 
material probably occurs at the base of some of the sills where it has segre- 
gated from the rest of the mass, like the picrite at the bottom of the 
Bathgate diabase. 
The amount of contact-metamorphism effected even by such thick sills 
as those of Trotternish and Shiant is much less than might be expected. It 
seldom goes beyond a mere induration of the strata for a few yards, often 
only for a few inches from the surface of junction. In the Shiant Isles, 
however, the shales between the sills have undergone a more remarkable 
alteration. They have not only been greatly indurated, but have acquired 
the globular or botryoidal structure so fully described by Macculloch. The 
spheroidal aggregates vary from not more than a line to more than half 
an inch in diameter, and appear on the surface as dark, irregularly grouped, 
pea-like aggregates. This structure is perhaps best developed immediately 
under the thick sill on the west side of Eilean Mhuire. 
The massive sills are not the only evidence of the injection of 
igneous material on the Shiant Isles. The sill, or more probably group of 
sills, forming Eilean Mhuire is traversed by a number of sheets of basalt 
varying from only two or three inches to 20 feet in thickness. These black 
fine-grained rocks invariably present chilled selvages next the coarse 
gabbro, and though they have been on the whole injected parallel to the 
general bedding or banding, they here and there break across it as veins. 
The most important of these later intrusions forms a columnar sill on the 
eastern side ol the island, and can be followed for several hundred yards. 
It consists of a dark finely crystalline olivine-basalt, which towards the 
margin assumes a dense black texture. Under the microscope Mr. Harker 
found a thin slice of this rock to be “ an olivine-basalt of semi-ophitie, 
semi-granulitic structure [7112], The olivine is mostly fresh, but part of 
it is converted into a yellowish-brown pseudomorph like iddingsite. Mag- 
netite occurs chiefly in imperfect octohedra. The felspar is in little 
lath-shaped sections, many of which are finely striated, and give extinction- 
angles indicating a labradorite. The augite, light brown in the slice, never 
has crystal-boundaries, and often enwraps the felspars.” 
The narrow veins are composed of a much closer-grained basalt in which 
a few scattered felspars are visible. Mr. Harker remarks, with regard to 
a thin slice of one of these rocks [7113], that “the microscope shows this, 
too, to be an olivine-basalt. The porphyritic felspars are twinned on the 
Carlsbad and albite laws. Olivine and pseudomorphs after it are well 
represented. Magnetite is only sparingly present. The general mass of 
the rock consists of very small striated prisms of labradorite, granules of 
augite, and interstitial matter which must be partly glassy.” 
This is perhaps the most striking of all the examples known to me 
where an older sill has been split open to receive a subsequent injection of 
molten material. The Eilean Mhuire gabbro must be at least 200 feet 
