CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 487 
embracing all the basalts, anamesites, dolerites, and diabases, and thus by far 
the most abundant rocks in the region ; 2d, Olivine-augite-serpentine Rocks, 
hitherto observed only in two localities ; 3d, Felspar-magnetite Rocks, consist- 
ing of the “ porphyrites,” which played so important a part among the earlier 
eruptions of the Carboniferous period; 4th, Felsitic Rocks, consisting of a 
few intrusive veins or dykes. 
A. Augite-Felspar Rocks. 
These crystalline masses play a chief part in the igneous phenomena of 
central Scotland. They present three recognisable types of structure, which 
however pass by insensible gradations into each other,—1st, The Diabases or 
granitoid type ; 2d, The Dolerites ; 3d, The Basalts. 
J. THe DIABASES OR GRANITOID TyPE. (Plate XI. figs. 1 and 2). 
1. General External Characters—The rocks embraced in this group are 
thoroughly crystalline in structure, and usually remarkably coarse in texture. 
Some of their individual crystals measure occasionally an inch in length. 
They vary in colour partly with the tint and proportion of the felspar, and 
partly with the degree of alteration which they have undergone. Some 
varieties have a mottled pink hue, from the flesh-coloured felspar ; most are 
more or less distinctly greenish, from the decay of their magnesian silicates, 
and consequent diffusion of saponite, delessite, or other secondary product. 
They are never amygdaloidal, nor ever porphyritic, though here and there 
erystals considerably larger than those of the general mass may be observed. 
They always occur as intrusive masses, generally in sheets or amorphous bosses. 
They have manifestly been intruded at some depth from the surface among the 
Carboniferous rocks, and have never flowed above ground in lava-streams. 
Examples of this type are found among the hills to the west of Edinburgh. 
Lindsay’s Craig near Kirkliston, Crossall Hill near Dalmeny, Muckraw near 
Torphichen, Auchensteary near Kilsyth, Croy, and the Carron Water above 
Denny, may be cited as localities where it is well displayed. But most of the 
coarsely crystalline intrusive sheets throughout the Lothians show the characters 
of this type more or less distinctly. 
2. Microscopic Characters.—The structure is essentially crystalline. In 
many slices no trace of any ground mass can be made out between the 
separate crystals. Here and there a small proportion of a dull, minutely 
granular or microfelsitic substance may be detected between crossed Nicols. 
Most of the rocks, however, have been considerably altered, and many alteration- 
products appear between the still recognisable original crystals. It is possible 
that there may have been at first in some of the varieties a glassy or felsitic 
