MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
163 
marking out the iudividuality of the quartz grains, and showing the 
wa}' in which they consolidated in independent masses. 
We will now proceed to illustrate our subject by noting the 
microscopical characters of so:n 3 of the chief minerals occurring as 
rock constitiisnts, pointing out S3m3 of the distinguishing marks by 
which they may be recognised. Let us first take Quartz. — Its appear- 
ance when it occurs as a base or in granite has been already noticed, 
but this mineral is frequently found distinctly crystallized, 
the crystals b^ing porphyritically embelded in a felspathic 
matrix. Such crystals may be at once known by their form, and by 
the magnificent unbroken colours displayed when polarised light is 
used. The peculiar concentric bordering of brilliant colours round a 
single coloured centre has already been mentioned as characteristic 
of quartz when found without a definite crystalline form, colours 
owing to the decreasing thickness of the edges. Cavities, some of 
them water ones, abound in it. 
Crystals of Felspar may be very easily recognised in most 
cases by their form and optical behaviour. The felspars are 
divided into two groups, according to their crystallographic system, 
the orthoclastic, in which the principal cleavages are at right angles 
to each other, and the plagioclastic, in which the planes of cleavage 
are at other than right angles. These latter are often termed triclinic, 
to distinguish them from the nionoclinic or orthoclastic group. Under 
polarised light it is as a rule not difficult to distinguish between the 
two groups. 
Sanidine is a transparent form which may be known from 
common orthoclase by its pure transparency, it is also frequently 
very distinctly crystallized. Sometimes, howv.'ver, its crystals will be 
found to enclose other minerals, such as augite, nepheline, magnetite, 
etc., and long needle-shaped crystals (belonites) are not unfrequently 
met with. Broken crystals of sanidine are often found enclosed in a 
trachytic base, from which we gather that they were already formed 
whilst the mass was in motion. 
Orthoclase, whilst presenting nearly similar forms to those 
of sanidine, differs from it in being very seldom clear. When 
polarised it often shows a remarkable irregular-banded structure, the 
