ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
7 
this pass to their minuter structure, and to the distinguishing 
microscopical characteristics of the principal minerals entering 
into their composition. The igneous rocks are divided into two 
great classes, the acid and the basic , the former containing a 
much higher percentage of silica than the latter. “ The former 
class is also sometimes known as the felspathic, orthoclastic, or 
trachytic series, and the latter as the augitic, plagioclastic, or 
basaltic.” To these two divisions must be added an interme- 
diate one, consisting of rocks forming intrusive masses, 
dykes, &c. 
The principal rocks in the acid series are, first, the crystal- 
line, granites, felsites, and felstones. The granites , consisting 
of quartz , orthoclase and oligoclase, felspars and mica, which is 
often replaced gradually in masses by hornblende, when the 
rock becomes syenitic. In these granites we may meet with 
various adventitious minerals, such as pyrites, marcasite, chalco- 
pyrite, garnet, apatite, and epidote, &c. Felsites and felstones 
are rocks very various in colour, composed of felspar and quartz, 
together with some easily decomposable mineral, which takes 
the place of the mica or hornblende of the granites. The 
granitic form of felsite is called eurite ; the compact varieties 
are known as felstones. The second class of rocks in the acid 
series is the glassy; the representatives are pitchstone and 
obsidian. The basic rocks also consist, first, of crystalline 
forms, the chief being gabbro, dolerite, anamesite, and basalt. 
Gabbro is composed of plagioclase felspar , frequently labrado- 
rite ; diallage , or some other pyroxenic mineral, such as hyper- 
sthene, or augite, and olivine. Amongst the adventitious 
minerals are magnetite, pyrites, marcasite, chalcopyrite, biotite, 
garnet, apatite, epidote, serpentine chlorite, nepheline, leucite, 
nosean, &c. When the diallage is replaced by augite, and the 
rock is granular, it is called dolerite ; finer grained varieties are 
anamesite and basalt. The glassy form of basic rocks is known 
as tachylite. 
Amongst the intermediate series of rocks which so frequently 
form intrusive masses or dykes we find diorite, composed prin- 
cipally of felspar and hornblende, and syenite ; porphyrite and 
phonolite are lavas which may also be classed with these. We 
may now turn to the structure of these rocks. We frequently 
find in them a vitreous or glassy base, enclosing crystalline 
minerals ; such a glass appears on a large scale in obsidian and 
in the pitchstones. A glass under the microscope presents to 
our view a perfectly structureless character, which possesses no 
trace of that double refraction when the polariscope is used, 
which is characteristic of all crystals except those of the cubic 
system, and therefore it is always dark between crossed prisms. 
64 Felspar may appear dark at the same time, but if the polariser 
