CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 507 
The most abundant mineral is a colourless and tolerably fresh triclinic felspar. 
Next in amount isa pale yellowish transparent mineral in very small prisms, which 
seems to be augite, but of an unusual form. Some of these prisms, not exceed- 
ing ‘0005 of an inch in diameter, may be seen enclosed in the altered olivine. The 
latter mineral can be recognised in the form of crystals, from about ‘03 to ‘10 
of an inch in length, completely serpentinised. They consist mainly of a pale 
delicate apple-green clouded and fibrous substance, which is bordered and 
traversed by strings of a bright grass green, sometimes of rich yellowish brown. 
But these olivines occur in much smaller quantity than in the lower part of the 
rock. Titaniferous iron or magnetite likewise appears as in that lower part, but 
also in less abundance. Apatite may be detected occasionally. (See fig. 6, 
Plate XI.) 
Without at present entering further into the detailed structure of this 
beautiful and interesting rock, the facts just stated show that in the lower half 
there is a preponderance of the heavy olivine, augite, and iron, while in the 
upper half the lighter felspar predominates. As I have said, it is quite 
impossible to draw any line between the two portions of the rock thus 
differently constituted. It is one indivisible mass, in which the lower part, 
a serpentine (representing olivine), shades up into the higher part, rich in 
felspar. In this case there has evidently been a separation of the ingredients 
according to their respective gravities, during the period when the mass was 
still in a molten condition, as ScroPeE pointed out in modern lavas. The fluidity 
of the rock must have been such as to allow of this segregation even after the 
lava had moved some way along the surface. 
The Inchcolm rock is considerably fresher than that of Blackburn. 
Examined under the microscope, it is seen to be by far the most beautiful rock 
in the Basin of the Firth of Forth. The olivine, its most abundant mineral, is 
still in large measure quite undecomposed, though frequently presenting the 
usual external crust and transverse wavy threads of green serpentine. Large 
pieces of fresh olivine give the characteristic reaction with polarised light. 
Next in quantity comes the augite, which is likewise singularly fresh. It 
has in thin slices a pale claret colour, and gives cleavage angles of 87° 
and 93°. It occurs in large well-defined prisms of the usual forms, often 
enclosing grains and crystals of olivine. A milky felspar, full of fissures, 
filled with decomposition products, but still showing traces of twin lamellation, 
occupies a very subordinate place in the rock. Long scales of rich brown 
biotite occur here and there; also a few plates and grains of probably titani- 
ferous iron. One of the most conspicuous ingredients is a rich emerald-green 
to grass-green product of decomposition, which fills up interstices, and running 
im veins and irregular streaks or tufts through the rock, gives a singularly 
bright tone to the field of the microscope. Other pale or colourless aggregates, 
