CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 503 
granulated augite, under a low power, appears as a pale port-wine tint ; but in 
thin slices, and with a high power, it is a pale brown or yellow, though the 
pink hue is not always lost even there. Perfect crystals of this mineral occur 
in some basalts; the most remarkable example in this respect with which I 
have met is from one of the dykes on the shore at St Monans, where, through a 
granular augitic base, regular prisms often twinned, are dispersed in great 
abundance, and admirably fresh. If minute petrographical subdivisions of the 
basalts were desirable, we might arrange in one series those where the felspar 
is greatly preponderant, and in another those where granular augite is the 
leading mineral. 
The olivine varies much in quantity. In some basalts it appears only in 
occasional rare and small pieces ; but usually it is discernible in every thin slice, 
and in some it forms a notable object even to the naked eye. It is always 
more or less converted into the usual green serpentinous substance. In certain 
basalts, especially in that of Mid Tartraven, Linlithgowshire, the olivine is 
occasionally so crowded with magnetite that this mineral forms by much the 
largest proportion of a crystal. The external form of the olivine remains 
distinct, however, and the altered serpentinous pseudomorph seems to bind the 
opaque octohedra together. In some of the Fife olivines the outer border is 
converted into a deep orange-yellow transparent strongly dichroic substance. 
The magnetic iron appears in two forms. In the great majority of the 
basalts it assumes its usual form of minute octohedra, the shining triangular 
faces of which are well brought out by reflected light. But in some peculiar 
basalts the iron exists as minute thin lamelle, which recall those of the 
diabases and coarse dolerites. From the angles obtained, however, and from the 
absence of the usual white porcelain-like accompaniment of titaniferous iron, 
I infer that these thin plates are magnetite. Basalts with this character 
occur at Pettycur in Fife, and also on the island of Inchkeith. Apatite is not 
recognisable. 
The alteration of the basalts has always begun by the conversion of the 
olivine. Where much of this mineral is present, the decay of the rock pro- 
duces the greenish hue so common among the interbedded sheets. The other 
minerals resist weathering for a long time, but the felspar is eventually kaolinised, 
the augite passes into a dark brown earthy substance, sometimes into serpen- 
tine, and the magnetite is oxidized into hematite, or more usually limonite. 
It will be seen that in general arrangement of structure, these Carboniferous 
basalts of central Scotland exactly correspond with felspar basalts described by 
ZiRKEL from Germany, Faroe, Iceland, and the United States,* as well as with 
those of Tertiary age collected by him in Scotland. The endeavour to establish 
* See his Basalt-Gesteine, 1870, and ‘‘ Report of Geol. Explor., 40th Parallel, Microseopical 
Petrography ” (vol. vi.), p. 229. Washington, 1876. 
