CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 495 
parts of one crystal with wedges of felspar between them, are then found to 
polarise on the same plane, and in short to belong to one undisturbed crystal 
which formed round and enclosed the already completed network of triclinic 
felspar prisms. Hence in one part of a slide we have one of these prisms 
completely enveloped in an augite crystal, while in an adjoining portion of the 
same preparation the augite seems to be completely enclosed within the felspar. 
There can be no doubt that, as above remarked, on the whole the felspar took 
crystalline form first. This is admirably shown in the contact-phenomena to 
which I shall afterwards refer. At the same time, there are occasionally 
indications that minute granules of augite in the original molten rock did form 
definite crystals before the crystallisation of the felspar was completed. 
There is one distinctive feature between the mode of occurrence of the 
augite in the dolerites and in the interbedded anamesites and basalts which I 
have found to hold good, with few exceptions. While in the intrusive sheets 
the augite occurs either in well-marked crystals or in large crystalline 
irregularly-shaped portions, in the superficial lava-beds it is commonly present 
in abundant small granules and in sparse definite crystals. The granulated 
form has never been noticed by me in the dolerites, save in a few cases where 
the rock may have been almost superficial, or where it has been rapidly chilled 
by contact with the rocks through which it has been intruded. This point 
will be further described in the section treating of the interbedded basalt- 
rocks. 
' Apatite is present in most of the rocks, though extremely variable in 
amount. It assumes the form of fine needles and of stouter hexagonal prisms, 
but never attains the size it reaches in the diabases. 
The opaque ferruginous mineral seems to be chiefly titaniferous iron. 
Distinct rhombohedral forms in thin lamellze may often be noticed, and not 
infrequently with the same tendency to build themselves into crystallographic 
forms, which I have already noticed. But minute grains showing octohedral 
faces occasionally reveal the presence of magnetite. 
Olivine is rarely recognisable, even as a serpentinous pseudomorph, and 
never in the freshly erystalline state. There is no greater contrast between the 
dolerites and the basalts than the part which this mineral plays in them respec- 
tively. In some dolerites small serpentine enclosures retain what are probably 
the outlines of former olivine grains and crystals. But these are small in size, 
and very vaguely characterised when contrasted with those which remain to 
be described in the basalt group. 
Some of the more salient characters of the dolerites are represented in fig. 3, 
Plate XI., which shows the structure of a portion of the rock of Dalmahoy 
Hill, Edinburgh. 
The alteration of the dolerites has produced results closely similar to those 
