12 
MUSEUM BULLETIN NO. 27 
diopside, the particular mineral formed depending, no doubt, on such 
factors as the composition of the fluids and on the temperature and 
pressure. It will be noted that these are all high temperature minerals, 
that do not readily crystallize except in the presence of a mineralizer 
such as would be afforded by the volatile constituents of the residual 
magma. 
Although granite and aplite dykes are quite common, traversing 
the peridotite and serpentine in all the asbestos and chromite pits, no 
reference is made in Dresser’s report to their displaying pegmatitic 
structure. True pegmatite dykes do, however, occur, although as a 
rule they are much altered, some of the feldspar being almost entirely 
converted into kaolin. Molybdenite, zircon, and tourmaline have been 
found in small quantities in such dykes at one or two localities, as noted 
below. Some of the coarse diopside “dykes” also have a structure 
comparable with that of pegmatite. 
According to the view here outlined, the “dykes” of lime-alumina 
silicates are closely related to the granite and aplite dykes, the fluids 
bearing these minerals in solution, or in aqueo-igneous fusion, having 
passed along the same series of fissures in the already solid portions of the 
magma, and crystallized out as soon as the necessary conditions of 
temperature and pressure were reached. 
Some aplite dykes, examined in thin section, were found to be highly 
garnetiferous, though others were more or less free from this mineral. 
Many were found to be traversed by cracks and veinlets cemented by 
fine granular to compact grossular garnet (Plate IX). This last feature 
might be taken as supporting the view that the granitic residue of the 
magma first assumed the solid state, and was later injected by the residual 
magmatic waters which had, in the meantime, become enriched in such 
constituents as lime as a result of their solvent action on the walls of the 
fissures. 
The dykes composed entirely of granular or massive garnet, such 
as those occurring at the Southwark pit and elsewhere, may then be 
regarded as the limiting phase of the above process, where, the granitic 
differentiate of the magma having all consolidated, the still fluid mag- 
matic waters have continued to flow along fissures and ultimately gave 
rise to dykes of garnet. At other localities, where different conditions 
obtained, vesuvianite or diopside appear instead of grossularite. 
MINERALS. 
Altogether, some thirty-four mineral species have been observed 
in the various asbestos and chromite pits in the area. These are described 
in the following pages, the sequence adopted being that of Dana’s 
System of Mineralogy. 
