ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
9 
we interpose a single refracting body between the prisms, we 
shall find that the field is still dark, or, in the case of certain 
crystals, illuminated only in a feeble but peculiar manner, 
suggesting a laminated structure in these crystals, which 
Biot has called “lamellar polarisation,” and striking in- 
stances of which are presented by leucite and boracite.* On the 
other hand, all double-refracting minerals between the crossed 
prisms allow the light to pass to the eye, and it is seen to be 
variously coloured, according to the thickness of the specimen 
under examination. There is, however, an exception to be 
borne in mind — viz. that sections of crystals cut at right angles 
to the optical axis have no double refraction in that direction, 
and therefore remain dark between crossed prisms. In the case 
of compound crystalline bodies, where in ordinary light we seem 
to have but a homogeneous substance, the polari scope will at 
once resolve it into its constituent parts, each being dis- 
tinguished sharply by its colour from its neighbours, showing, 
to use the convenient German term, “ aggregate polarisation.” 
We may now proceed to the microscopic appearances of the 
various minerals.! Let us begin with those most commonly met 
with, and first — Quartz. I have already had occasion to notice 
the appearance of quartz when it occurs as a base, but we very 
frequently meet with this mineral distinctly crystallised ; some- 
times the crystals are large and porphyritically embedded in a 
felspathic matrix. Such crystals may be at once known by 
their form, and by the magnificent unbroken colours they dis- 
play when polarised light is used. The peculiar concentric 
bordering of brilliant colours round a single coloured centre, 
mentioned in speaking of a quartz base, is very characteristic 
of this mineral whenever it is met with without any distinct 
crystalline form, and is owing to the decreasing thickness of the 
individual masses around their edges. Cavities of all sorts are 
abundant in quartz. 
Felspar . — The various species of felspar present striking 
differences when examined microscopically, and the monoclinic 
forms are readily distinguished from the triclinic, as sanidine 
and orthoclase from plagioclase and labradorite. 
Sanidine . — Sanidine is usually seen, in thin sections, either 
well crystallised or in crystalline grains and fragments; its 
forms when perfectly crystallised are various but characteristic. 
Sanidine is generally very pure and transparent, though occa- 
sionally it encloses other minerals, such as plagioclase, nephe- 
* In the case of Boracite this lamellar polarisation has been considered to 
he owing to interposed lamina of a double-refracting substance, slightly dif- 
fering from it in chemical composition, which M. Yolger has called parasite. 
t Authorities, Rosenbusch, Zirkel, &c. 
