494 GEOLOGY. 



rock. These rocks are very simple in their nature, being gene- 

 rally composed of not more than five minerals, viz. quartz, fel- 

 spar, mica, hornblende, and limestone. Some rocks are com- 

 posed of but one of these simple minerals, as quartz rock ; others 

 of two, such as mica- slate, which is a compound of mica and 

 quartz ; and others, as granite, consist of three, quartz, felspar, 

 and mica. An intimate acquaintance with these five simple 

 minerals, and with the appearances they assume in aggregated 

 mountain rocks, will enable the student to determine their species. 

 The primitive rocks are thus characterized by Professor Jame- 

 son : 



" 1. Granite is a granular compound of felspar, quartz, and mica ; 

 syenite is a variety of granite, containing, besides the ingredients al- 

 ready enumerated, also hornblende. 



" 2. Porphyry is an aggregate rock, having a basis or ground con- 

 taining imbedded grains and cyrstals of felspar, and sometimes of 

 quartz and hornblende. 



" 3. Trap. All the rocks of the primitive class in which horn- 

 blende is the predominating ingredient are named trap. On exposure 

 to the air they assume the form of steps of a stair, hence the name 

 trap. When the hornblende is associated with felspar, it forms 

 greenstone ; if unmixed, hornblende rock ; and if slaty, hornblende 

 slate. 



" 4. Serpentine is a dark green rock, with a splintery fracture, and 

 glimmering or dull lustre, translucent on the edges, and so soft as to 

 yield readily to the knife. It is conjectured to be a compound of 

 felspar, and of a mineral of the nature of hornblende, named dial- 

 lage. 



" 5. Limestone. This rock has generally a white or gray colour, is 

 composed of shining granular concretions, and is more or less trans- 

 lucent. It frequently contains scales of mica and grains of quartz. 



" 6. Gneiss is a granular slaty compound of felspar, quartz, and 

 mica. 



" 7- Mica-slate is a slaty compound of mica and quartz. 



" 8. Clay-slate is a slaty rock, generally composed of extremely 

 minute scales of mica. It is the roof slate so well known in the arts. 



" 9. Quartz rock. This rock is almost entirely composed of 

 quartz, either in granular concretions, or in the compact state ; and 

 grains of felspar and scales of mica are frequently contained in it." 



II. TRANSITION ROCKS. Transition rocks succeed to the 



