THE GEOLOGIST. 
FEBRUARY, 1859. 
ON ROCKS ; THEIR CHEMICAL AND MINERAL COM- 
POSITION, AND PHYSICAL CHARACTERISTICS. 
By H. C. Salmon, Esq. Plymouth. 
{Continued from Vol. I. page 420.) 
2. On the various chemical and mineral constituents of rocks, and their 
general relations. 
IX. All rocks are necessarily composed of minerals, — that is, of 
certain " substances which, wherever found, present respectively 
nearly the same forms and physical characters, and are generally 
composed of nearly the same chemical constituents." * A rock may 
consist of one single mineral, in which case it is called a simple rock ; 
or it may be made up of an aggregation of several different minerals, 
when it is called a mixed rock. Crystalline limestone, which consists 
exclusively of one mineral, calcite, may be given as a familiar example 
of a simple rock, and granite, made up of an intimate aggregation of 
three distinct minerals, felspar, quartz, and 7nica, of a mixed rock. 
Rocks being thus made up of minerals, it will be necessary for us 
to consider briefly the composition and classification of the latter. 
They are divided by mineralogists into species, and these species are 
again grouped together according to various systems of classification. 
* Phillips's Mineralogy, edited by Brooke and Miller. " A mineral species is a 
natural inorganic body, possessing a definite chemical composition, and assuming 
a regidar deteminate form or series of forms. There are, however, certain 
limitations with which the above definition must be understood." — Nicol's 
Mineralogy. 
VOL. II. E 
