CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 489 
showing that the augite has crystallised later than the felspar. Asa rule the 
augite is tolerably free of extraneous substances, It is usually much cleaved 
‘and cracked, with occasional cavities either empty or filled with some decom- 
position product. Its most frequent endomorphs are needles and distinct 
hexagonal prisms of apatite. Thin rhomboidal plates of titaniferous iron are 
also occasionally to be noticed. The colour of the augite varies from a pale 
port-wine tint to a pale greenish brown in the fresh state. Here and there, 
even in the comparatively unchanged mineral, layers of a dirty green or brown 
\ 
decomposed substance, may be noticed in the internal cavities, or coating with 
a thin film the sides of the minute fissures. This is doubtless a product of 
decomposition of the augite itself. Every stage may be followed, from the first 
appearance of change until all trace of recognisable augite has disappeared, its 
place being taken by a mass of opaque brown and green amorphous earthy 
matter. In some varieties of rock the augite almost disappears. This may be 
observed in portions of the intrusive sheet west of Denny, where the rock 
consists mainly of or'thoclase. 
Olivine has not been certainly detected by me in any rocks of this type. 
Its presence may indeed be suspected from the serpentine so frequently 
to be noticed among the ingredients. But in no case have I ever observed 
any definite boundaries to the serpentine enclosures such as to indicate the 
outlines of olivine crystals. This is the more remarkable, as in many of the 
dolerites of the region which are quite as much altered as the rocks now under 
description, the olivine, though entirely serpentinised, still shows very distinctly 
the original outline of its crystals and granules. If any olivine ever existed in 
these coarsely crystalline intrusive rocks it must, I think, have been in very 
small quantity, and probably in granular indefinite aggregates rather than in 
homogeneous defined crystals. 
Titaniferous iron is always present, and often in most interesting forms. 
Its fragmentary rhombohedra are scattered abundantly throughout the sub- 
stance of the rock, sometimes enclosed in the augite and felspar, but usually 
independent. It frequently assumes the form of thin rhombohedral plates, 
which may now and then be observed to lie apart but parallel with each other, 
in obedience to original crystallographic force. There is a tendency not seldom 
to be observed, in these detached plates, to build themselves together into the 
framework of a crystal. Beautifully perfect skeleton rhombohedra of this 
kind may be seen in the rock of Crossall Hill, near Dalmeny (see fig. 26). 
Viewed by transmitted light, no structure can be made out in the individual 
crystals or lamelle, though here and there they may be seen associated 
_ with the peculiar milky white /ewcoxene which sometimes, indeed, as DE La 
VALLEE Poussin and Renarp have shown in the case of the Belgian plutonic 
rocks, replaces the opaque iron in part or the whole of a crystalline face. 
VOL. XXIX. PART I. 6K 
