
108 -  G@EOGNOSY.” ‘- [Book I. 
ferruginous minerals. Opacite—black, opaque grains and scales of 
amorphous earthy matter, which may in different cases be magnetite, 
or some other metallic oxide, earthy silicates, graphite, &e.* 
§ V.—Classification of Rocks. 
It is evident that lithology may be approaehed. from two very 
different sides. We may on the one hand regard rocks as so many 
masses of mineral matter, presenting great variety of chemical 
composition and marvellous diversity of microscopic structure. Or 
on the other hand, passing from the. details of their chemical and 
mineralogical characters, we may look at them, as the records of 
ancient terrestrial changes. In the former aspect, they present for 
consideration problems of the highest interest in inorganic chemistry 
and mineralogy; in the. latter view they invite attention to the great 
geological revolutions through which the planet has passed. It is 
evident therefore that two distinct systems of classification may be 
followed, the one based on chemical and mineralogical, the other on 
geological considerations. 
From a chemical point of view, rocks may be grouped according 
to their composition; as odes, exemplified by formations of quartz, 
hematite, or magnetite; carbonates, including the limestones and 
clay-ironstones ; szlicates, embracing the vast majority of rocks, 
whether composea of a single mineral, or of more than one; phosphates, 
such as guano and the older bone beds and coprolitic deposits, A 
classification of this kind, however, pays no regard to the mode of 
origin or conditions of occurrence of the rocks, and is quite unsuited 
for the purposes of the geologist. 
From the mineralogical side, rocks may be classified with reference 
to their prevailing mineral constituent. Thus such subdivisions as 
Calcareous rocks, Quartzose rocks, Orthoclase rocks, Plagioclase rocks, 
Pyroxenic rocks, Hornblendic rocks, &c., may be adopted ; but these 
are hardly less objectionable to the geologist, and are in fact suited 
rather for the arrangement of hand-specimens in a museum, than 
for the investigation of rocks 7 situ. 
From the standpoint of geological inquiry, rocks have been 
classified according to their mode of origin. In one system they are 
arranged under three great divisions: 1st, Igneous, embracing all 
which have been erupted from the heated interior of the earth ; 
2nd, Aqueous or Sedimentary, including all which have been laid 
down as mechanical or chemical deposits from water or air, and 
all which have resulted from the growth and decay of plants or 
animals; 3rd, Metamorphic, those which have undergone subsequent 
change within the crust of the earth, whereby their original character 
has been so modified, as to be sometimes quite indeterminable. An- 
other geological arrangement is based upon the general structure of 
' Vogelsang, Z. Deutsch. Geol. Ges, xxiy. (1872) p, 529, Zirkel, Geol. Expl, 40th 
Parallel, yol. vi. p. 12, 
