490 PROFESSOR GEIKIE ON THE 
When, however, reflected light is employed, the velvet-black colour of the 
titaniferous iron, its semi-metallic lustre, small con- 
choidal fracture, and cleavage lines can be admirably 
seen. By this means also we ascertain that many 
apparently homogeneous masses of titaniferous iron 
have cavernous centres, more or less completely 
filled with pyrite. Amorphous particles of the same 
iron oxide also occur. Possibly these may conceal 
some magnetite. But the latter mineral has not been 
identified in any determinable crystalline forms among’ 
these rocks. 
Ween eee ee Apatite is probably never wholly absent, though 
of titaniferous iron, Crossall Hill, its proportions vary within remarkably wide limits. 
Dalmeny (magnified 20 diameters). Tn such very coarsely crystalline rocks as that of 
Crossall Hill, it occurs in colourless but somewhat dusty stout hexagonal 
prisms, sometimes 2°5 millimetres in diameter. It may there be seen shooting 
across from an augite into a felspar crystal without fracture, whence we may 
infer that at least in these portions of the rock there could have been but little 
motion of the mass at the time of crystallisation of the minerals. In other 
varieties of rock, such as that of Ratho, the apatite is crowded in the form of 
minute and perfectly clear hairs through the felspar. 
In one or two examples quartz appears as an original constituent, in the 
form of small blebs. This is well shown in the case of the remarkable sheet of 
intrusive rock which has invaded the Carboniferous Limestone series in tlie 
valley of the Carron Water above Denny. The quartz appears there as occa- 
sional clear particles between the abundant kaolinised felspars, decayed 
titaniferous iron, and sparse, much altered augite. It contains numerous fluid 
cavities and small microlites. In most cases however, quartz, when present, is 
full of clouded impurities, contains no fluid cavities, and is aggregated in 
forms characteristic of this mineral as a secondary product. 
As the rocks of the type now being described have all undergone more or 
less alteration, secondary products abound in them. Many of these form con- 
spicuous features in the large veins and cavities to be seen in the quarry or 
natural section. Calcite, prehnite, pectolite, analcime, have long been known 
from these masses, and fine specimens, particularly of pectolite, have been 
obtained from Ratho and Corstorphine. Examined under the microscope, 
nearly every slice exhibits more or less of the amorphous brown and green 
earthy substance already referred to. This may be seen investing the augite 
and penetrating its fissures. But it also occurs diffused through rocks im 
which this mineral still remains fresh. It is not, therefore, produced by all 
varieties of augite, and is no doubt in a large number of instances due to the 
