332 
THE TERTIARY VOLCANOES 
BOOK VIII 
makes them conspicuous by contrast with the dark brown or black hue of 
the rocks which they traverse, and shows at once that they must be poorer 
in bases than these. They are found on microscopic examination to consist 
ot the same minerals as the more coarsely crystalline gabbros, hut with a 
much greater abundance of the felspar. They contain also apatite, and 
hornblende appears to predominate in them over augite. They are to be 
distinguished from the pale veins that form apophyses from the intrusive 
granophyres. 
Troctolite (Forellenstein). — This beautiful variety of plagioclase- 
olivine rock occurs as a conspicuous feature on the east side of the gabbro-area 
of the island of Rum. It forms a sill on the side of the mountain Allival, in 
which the component minerals are drawn out parallel with the upper and 
under surfaces of the bed (Fig. 341). So marked is this flow-structure that 
hand-specimens might readily be taken at the first glance for ancient schistose 
limestone. “ The felspathic ingredient (probably labradorite or anorthite) is 
white, and its lath-shaped crystals have ranged themselves with their long 
axes parallel to the line ot flow. The olivine occurs in perfectly fresh grains, 
which in hand-specimens have a delicate green tint. Under the microscope 
they appear colourless, and are penetrated by the felspar prisms in ophitic 
intergrowth. There is a small quantity of a pale brownish augite, which 
not only occurs in wedge-shaped portions between the felspars, but also as 
a narrow zone round the olivines.” 1 Considerable differences are visible in the 
development of the flow-structure, and with these there appear to he accom- 
panying variations in the microscopic structure. Dr. Hatch, to whom I 
submitted my specimens, informed me that in one of them, where the flow- 
structure is so marked as to give a finely schistose aspect to the rock, “ there 
is a larger proportion of augite, some of which exhibits a distinct diallagic 
striping ; the olivine grains show no ophitic structure, but are sometimes 
completely embedded in the augite.” To this remarkable flow-structure I 
shall again refer in connection with the light it throws on the bedded 
character of much of the gabbro bosses. 
Between the different basic intrusive igneous rocks of the Inner Hebrides, 
as Professor Judd has shown, there are many gradations according to the vary- 
ing proportions of the chief component minerals. Thus from the olivine- 
gabbros, by the diminution or disappearance of the augite we get such rocks 
as troctolite ; where the plagioclase diminishes or vanishes, we have different 
forms ot picrite ; where the olivine is left out, we come to compounds, like 
eucrite ; while by the lessening or disappearance of the felspar and augite, 
we are led to ultra basic compounds, consisting in greatest part of olivine, 
like lherzolite, dunite and serpentine. To some of the features and probable 
origin of these chemical and mineralogical diversities in the same great 
eruptive mass further reference will be made in later pages. 
1 MS. of Dr. Hatch. 
