MINERALOGY OF BLACK LAKE AREA. 
31 
White, compact diopside rock occurs here as dyke-like bodies cut- 
ting the serpentine, and also in the form of narrow stringers and veinlets 
in the massive chromite; its mode of origin has already been discussed 
in an earlier section. Upon the walls of fissures and drusy cavities 
within this massive diopside rock, where free crystallization has been 
possible, the mineral appears as small, but well formed, colourless, trans- 
parent crystals. 
A thin section of the massive rock photographed in ordinary light is 
shown in Plate VIII A. The rock has a granitic texture, and examina- 
tion shows it to be composed entirely of diopside, with the exception of 
a very little calcite or dolomite. The diopside is colourless, and very 
fresh and clear; it forms fairly large interlocking idiomorphic individuals, 
and also granular aggregates of smaller crystals, which fill the inter- 
stices between these. Small druses are seen here and there, and it is in 
some of these that the carbonates have been deposited. 
A second variety of diopside is found in which the mineral forms 
large tabular crystals and platy masses with a white, or often pale lilac, 
colour (the latter due to the presence of manganese). This coarsely 
crystallized material appears to be closely related to the other, and it 
often occurs in the same way quite close to the serpentine, from which 
it is separated only by a narrow zone of the compact white diopside. 
It seems reasonable to suppose that, under certain conditions, the 
solutions from which the diopside crystallized were cooled so rapidly in 
making their way along the fissures in the serpentine, that they were 
continually supersaturated, and the diopside crystallized out as a shower 
of minute crystals; this might be expected in the narrow fissures especi- 
ally and it would give rise to a “dyke'’ of compact diopside rock. Resid- 
ual solutions, filling cavities and druses in such rock, would then cool 
more slowly, with the formation of the colourless transparent crystals. 
Where the fissures in the serpentine were wider, however, different 
conditions would obtain; in such cases, the solutions would become 
highly supersaturated only where they were chilled by the serpentine 
walls. Elsewhere any crystals which formed might continue to grow 
until they ultimately formed a network stretching across the fissure. 
It is believed that the large tabular crystals and platy masses have 
originated in this way. 
Colourless Crystals . 
The colourless crystals are remarkable for two features: in the first 
place the occurrence of diopside in absolutely colourless and transparent 
crystals is in itself somewhat of a rarity; in addition to this, the majority 
of the crystals present a habit, due to the large relative size of the faces 
