320 
THE TERTIARY VOLCANOES 
BOOK VIII 
of pitchstone lies among the bedded basalts on the east side of the 
hill. 
From a number of specimens collected by me during a second visit to 
this district in the summer of 1896, I selected some for microscopic ex- 
amination and submitted them to Mr. Harker, who has furnished me with 
the following descriptions of them : “ The sill at the north end of Camas 
na Cloiehe, Ben Hiant [7114] is an olivine-gabbro of medium grain and 
fresh appearance. Olivine, fresh or partly serpentinized, is plentiful. The 
felspar is a labradorite with Carlsbad- and albite- (rarely pericline-) twinning, 
and some of it has zonary banding. It is for the most part in crystals 
giving rectangular sections, but there are some of allotriomorphic form. 
Magnetite occurs chiefly in shapeless grains of later crystallization than the 
felspar, but sometimes presenting crystal-faces to the augite. The augite 
is light-brown in the slice, without any true diallage-structure, and tends 
to enwrap the earlier minerals in ophitic patches. 
“ The sill south of Uamh na Creadha, on the west side of Ben Hiant 
[7115], is a rock of different type, having porphyritic crystals of felspar, up 
to an inch or more in length, in a rather finely-crystalline groundmass. 
The microscope shows it to be a dolerite of granulitic structure, the main 
mass of the rock consisting of little striated labradorite-crystals, grains of 
pyroxene, and rather abundant crystal-grains of magnetite. The pyroxene 
seems to be chiefly augite, but hypersthene is also present, and builds rather 
larger and more idiomorphic crystals with characteristic pleochroism.” 
In rambling over this Ardnamurchan district I have often been re- 
minded of the great intrusive sheets of Fair Head. One of the features in 
which the rocks of the two localities resemble each other is their tendency 
to assume a coarsely crystalline texture. In some parts of Ben Hiant the 
individual crystals reach an inch or more in length. These more largely 
crystalline portions, however, do not form distinct bands so much as patches 
in the midst of the general mass ; at least I have not noticed any examples 
of such veins of segregation as are so prominent in Antrim. 
Ho one familiar with the well-marked distinctions between the lavas of 
the plateaux and the sills which traverse them can hesitate in which series 
to place the rocks of Ben Hiant. Since, however, these rocks have been 
claimed by Professor Judd as the superficial lava -currents of a volcano 
which broke out after the time of the plateau-basalts, like the Scuir of 
Eigg, some further details in regard to the geological structure of the 
district, which would otherwise be superfluous, may here be given. 
The number of sills and dykes iu Ardnamurchan is astonishingly great. 
There must be hundreds of them visible, and perhaps as many more 
concealed under superficial coverings. They are well exposed on the shore 
traversing the Jurassic strata and the schists. The sills become especially 
large and abundant in the direction of Ben Hiant, which has evidently 
been the principal centre from which their materials were injected. 
The rocks composing these sills are quite similar to those of Ben 
Hiant, save that, as they occur in thinner sheets than in that moun- 
