CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 501 
(1.) The more distinctly crystalline basalts or anamesites are well illustrated 
by the rocks of Craiglockhart Hill (see fig. 4, Plate XI.), and the Long Row, 
near Edinburgh. When a thin section of one of these rocks is placed under 
the microscope, the first mineral to arrest attention is the abundant and fresh 
triclinic felspar. It forms the predominating ingredient, occurring in prisms 
with perhaps an average thickness of 002 inch, and a length of about -03 to 05 
inch. Occasional large porphyritic crystals ‘1 inch and upwards in length may 
be seen. This mineral has frequently enclosed globular grains of augite, and 
minute octohedra or irregular particles of magnetite. With a high power it 
may be seen to be sometimes full of fine clear glassy spicules. Beautiful fluid 
structure is shown by the arrangement of the thin felspar prisms round some of 
the larger included crystals. 
The augite comes next in abundance to the felspar ; indeed, it sometimes 
equals if it does not actually exceed it inquantity. This mineral is likewise well 
preserved. It occurs in two distinct forms :—1st, As minute granules without 
crystalline contours, but with the rounded drop-like form so characteristic of many 
basalts. This is the predominant condition. 2d, As large admirably definite 
prisms of the customary forms. ‘These, as previously stated, are sometimes half 
an inch long, and can be seen projecting from some of the weathered faces of the 
rock at Craiglockhart Hill. The zonal growth structure is beautifully displayed 
by some of these augites. I have observed in a few instances, that though the 
external form is sharply defined by a continuous band of the mineral, the 
interior presents a granulated appearance, as if it consisted merely of a congeries 
of the augite granules. In the Craiglockhart rock some of the augites may be 
seen nearly unflawed, while others immediately adjacent are entirely granulated. 
The latter portions, however, polarise uniformly as a whole, and not according 
to the individual granules. Small crystals of felspar, grains of magnetite, and 
portions of the ground-mass may be observed between the granules, which are 
of larger size than the average of those in the ground-mass. 
The olivine is almost always readily apparent. In some of the rocks it 
appears in crystals easily seen with the naked eye, some of them indeed, as at 
Craiglockhart, reaching a length of more than 4 inch. I have never observed 
it in small grains like the augite. It occurs in two conditions—(a) In sharply 
defined crystals, with easily measurable angles. When in this form, it contains 
few or no endomorphs, and has been able to resist alteration better than in the 
other form. Some of its central portions may be found still clear, and showing 
the proper reaction with polarised light. But its borders are altered into a pale 
greyish or greenish white substance which also traverses the centre along 
fissures. This substance polarises like serpentine, and is doubtless a serpen- 
tinous alteration-product. (6) In the usual, somewhat rounded, ill-defined 
erystals and grains. In this condition the olivine is often full of magnetite 
VOL. XXIX. PART I. 6N 
