1915.] NATURAL SCIENCES OF PHILADELPHIA. 427 



stituents, and not to have a strict quantitative significance, although 

 in general the ''silicic" rocks contain more than 45% Si02, while 

 in the other three over 7.5% of the oxides to which the name of 

 each refers i& usually present. 



The classification has been extended to about 800 mineral species, 

 some rarf! minerals being necessarily omitted because of lack of 

 information as to their occurrence, although it is to be expected that 

 many additions to the lists will prove necessary as our knowledge of 

 mineral associations advances. 



Much of the data has been obtained from recent works, especiallj^ 

 those of Lindgren,- Emmons,^ and Rogers.* 



SYNOPSIS OF THE CLASSIFICATION 

 I. Magmatic Phenomena 



1. Igneous rocks 



A. Silicic (comprising acidic and intermediate, but excluding 



alkalic, of the usual classifications) 



B. Alkalic (alkali-syenites and similar rocks) 



C. Calcic (the basic rocks) 



D. Magnesic (the ultra-basic rocks) 



Each of the above divisions is subdivided as follows : 



a. Primary 



b. Metamorphosed 



c. Weathered 



2. Pegmatites (including pneumatolytic veins and many quartz veins) 



A. Sihcic 



B. Alkalic 



C. Calcic 



Each of the above divisions i& subdivided as follows : 



a. Primary 



b. Metamorphosed 



c. Weathered 



3. Hydrothermal deposits (the majority of mineral veins, including 



contact deposits) 

 (No chemical subdivision practicable) 



a. Primary 



b. Metamorphosed (including secondarily enriched) 



c. Weathered 



■■' Econ. Geol., Vol. 2, p. 105, 1907. 

 ^Ecou. Geol., Vol. 3, p. 611, 1908. 

 *Econ. Geol., Vol. 7, p. 638, 1912. 



