MELLO: MICROSCOPICAL STRUCTURE OF R<X'KS 
are, although separated by a vast chi'onological interval, and we shall 
be able to correlate the older igneous rocks of palaeozoic age with 
the tertiary anamesites, basalts, and dolerites, and even with the 
products of still active volcanoes. 
We will now turn to some of the chief peculiarities of the 
igneous rocks, with a view to their microscopical examination. The 
igneous rocks may be divided chemically into two great classes, the 
acid, and the basic, the former containing 60 per cent, and upwards of 
silica, a much liigher per-centage than the latter. The felspar 
in the former is chiefly orthoclastic, in the latter it is plagioclastic. 
The principal rocks in the acid series are, hrj^t, the crystallino, 
such as gi'anite, felsite or felstone, and trachyte, etc. The granites 
consist of quartz, felspar (orthoclase and oligoclase) and mica, 
which latter mineral is often replaced gradually by hornblende, 
and when this is the prevailing element the rock becomes 
syenitic. In granite we may meet with various adventitious minerals, 
some the result of changes going on since the rock was formed; 
amongst the adventitious minerals will be found pyrites, marcasite, 
chalcopyrite, garnet, apatite, epidote, and occasionally calcite, etc. 
Felsites and felstones are rocks very various in colour, composed of 
quartz and felspar (orthoclase) together with some easily decompos- 
able 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 division of rocks in the 
acid class is the glassy, the representatives are pitclistone and obsidian. 
The basic rocks also consist, first, of crystalline forms, the chief 
■ being gabbro and dolerite, and its varieties anamesite and basalt ; 
gabbro is composed of plagioclase felspar, frequently labradorite, 
known by its opalescence when occuring in masses, diallage or some 
other pyroxenic mineral, such as hypersthene or augite, and oHvinc. 
Amongst the adventitious minerals are magnetite, pyrites, marcasite, 
chalcopyrite, biotite, garnet, apatite, epidote, serpentine, chlorite, 
nepheline, nosean, leucite, calcite, etc. 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 
