CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 509 
ing has particularly attacked the iron oxide in the rocks, hence the frequent 
red, brown, and yellow tints. 
An important feature of the porphyrites as compared with the basalts is 
their comparative lightness. Their average specific gravity is about 2°6 to 2°7, 
while that of the basalts is about 2°9, a difference which is appreciable even 
when specimens are held in the hand, Some intermediate varieties, however, 
helping to connect the porphyrites with the basalts, are not always easily dis- 
tinguished by external tests. 
2. Microscopic Characters.—The distinguishing mark of the porphyrites 
under the microscope is the character of their ground-mass. It appears as a 
clear, colourless substance through which vaguely defined prisms of triclinic 
felspar are crowded. Between crossed Nicols it presents a characteristic 
mottled structure, the light and dark parts shading off insensibly into each 
other. As the slide is rotated the mottling wanders over it, every portion 
becoming successively light and dark (see Plate XII. figs. 7 and 8). This 
continues to be the case even with a high power. In a very few cases only 
have I noticed small interspaces which remained persistently dark. Colourless 
hairs or fine rods are not infrequent ; and occasionally minute pale yellow or 
nearly colourless globules, which polarise like the globular augite of the basalts, 
may be observed. 
This clear anisotropic ground-mass can scarcely be anything else than a 
felspar. It blends so insensibly with the felspar prisms, that where the defined 
forms of these prisms cease it is impossible to separate their substance from 
the surrounding mass. This is the case with most of the Garlton Hill 
porphyrites. In some cases the prisms are well striated, and stand out more 
definitely. I have attempted to delineate the structure in the drawings above 
referred to. 
The felspars chiefly occur in these vaguely outlined forms. But in many 
porphyrites they appear also as large porphyritic crystals with exceedingly 
sharply marked boundaries. In the majority of cases they are triclinic, and 
probably labradorite. Now and then clear twins of orthoclase are to be 
observed. Among the Garlton Hills the larger felspars are occasionally 
crowded with enclosures, sometimes promiscuously diffused through the 
crystal, at other times in lines along the planes of twinning. The most 
abundant enclosures are minute globules of augite, not infrequently elongated 
into rod-like bodies. In one or two cases I have noticed numerous perfectly 
black opaque globules. These may be augite crowded with magnetite dust. 
Taking the clear ground-mass as felspar, and including with it the recognis- 
able felspar crystals, we find that perhaps about nine-tenths of the substance 
of one of the most typical porphyrites is felspathic. Next in abundance 
is magnetite, which occurs in recognisable octohedra, not infrequently 
VOL. XXIX. PART I. 6 P 
