ORIGIN OF CRYSTALLINE ROCKS. 491 
itself with species which are generally isomorphous or home@omor- 
phous, and of related chemical composition. Allied to this is the 
repeated alternation of crystalline laminz of related species, as 
in perthite, the crystalline cleavable masses of which consist of 
thin, alternating layers of orthoclase and albite. 
Very unlike to the above are those cases of envelopment in 
which no relations of crystalline symmetry nor of similar chemi- 
cal constitution can be ‘traced. Examples of this kind are seen in 
garnet crystals, the walls of which are shells, sometimes no 
thicker than paper, enclosing in different cases, crystalline carbon- 
ate of lime, epidote, chlorite or quartz. In like manner, crystal- 
line shells of leucite enclose feldspar, hollow prisms of tourmaline 
are filled with crystals of mica or with hydrous peroxyd of iron, 
and crystals of beryl with a granular mixture of orthoclase and 
quartz, holding small crystals of garnet and tourmaline, a compo- 
sition Hentieal with the enclosing granitic veinstone.* Similar 
shells of galenite and of zircon, having the external forms of 
these species, are also found filled with calcite. In many of these 
cases the process seems to have been first the formation of a hol- 
low mould or skeleton-crystal (a phenomenon sometimes observed 
in salts crystallizing from solutions), the cavity being subsequently 
filled with other matters. Such a process is conceivable in free 
crystals found in veins, as for example, galenite, zircon, tourmaline, 
beryl and some examples of garnet, but is not so intelligible in the 
case of those garnets imbedded in mica-schist, studied by Delesse, 
which enclosed within their crystalline shells irregular masses of 
white quartz, with some little admixture of garnet. Delesse con- 
ceives these and similar cases to be produced by a process analogous 
to that seen in the crystallization of calcite in the Fontainebleau 
sandstone ; where the quartz grains, mechanically enclosed in well- 
defined rhombohedral crystals, equal, according to him, sixty-five 
per cent., of the mass. Very similar to these are the crystalloids 
with the form of orthoclase, which sometimes consist in large part 
of a granular mixture of quartz, mica and orthoclase, with a little 
cassiterite, and in other cases, contain two-thirds their weight of 
the latter mineral, with an admixture of orthoclase and quartz. 
Crystals with the form of scapolite, but made up, in a great part, O of 
mica, seem to be like cases of envelopment, in which a small pro- 
portion of one substance in the act of crystallization, compels in- 
* Report Geol. Survey of Canada, 1866, page 189. 
