THE GEOLOGIST. 



FEBRUARY, 1859. 



OX 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 minei^al constituents of rochs, 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 mica, 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 MUler. " A mineral species is a 

 natural inorganic body, possessing a definite chemical composition, and assuming 

 a regular determinate form or series of forms. There are, however, certain 

 limitations with which the above definition must be understood."— Nicol's 

 Mineralogy, 



YOL. II. E 



