

CHAPTEE IL 



ROCKS. 



Kocka classified. Superposition of stratified rocks. Lamination. 

 Stratification. Denudation. Cleavage. Joints. The condition 

 under which metal-bearing deposits are found. Nature of mineral 

 veins in a lode, &c. Dip. Strike. Clinometer. Compass. 



THE following are the various divisions under which rocks 

 may be classified : 



IGNEOUS. (Rocks which have been subjected to heat.) 



Volcanic (those that have been cooled at or near the surface) : 



Trachyte (rough, greyish in colour, and light in weight). 



Basalt (blackish or brown, heavier and with fewer holes in it 

 than trachyte). Phonolite, Andesite (of which porphyrite 

 is an altered variety). Dolerite (with crystals more promi- 

 nent than in basalt) : el vans (including quartz -porphyry) : 

 pitchstone, &c. These last three occur as dykes or intru- 

 sive sheets : the two last are offshoots of granite formations. 



Obsidian (usually transparent and like bottle glass, pumice, 



&c.) : rhy elite, &c. 



Plutonic (those that have cooled at some depth below the sur- 

 face) : 



Granite, porphyry, syenite, diorite, gabbro, &c. ; these usually 

 have a distinctly crystalline structure, frequently with large 

 crystals. 



METAMORPHIC. (Of igneous and aqueous origin, but which have 

 undergone a change by pressure, &c.) 



Gneiss (in composition like granite, but foliated). 



Mica schist (quartz and mica), hornblende schist, talc schist, chlo- 

 rite schist, diorite schist, are some of the foliated forms. 

 Quartzite and some serpentines are metamorphic. 



A.QUEOUS. (Deposited by liquid agency.) 



Gravel (made up of loose rounded pebbles), conglomerates and 



breccias. 



Grit (in which the grains, usually of quartz, are cemented together). 

 Sandstone (in which quartz grains are very fine). 



