778 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



different chemical compositions. If the roof of a batholith is fissured some 

 time after its abyssal injection has taken place, floods of liparitic (rhyolitic) 

 lava result. The possibility is recognized that certain areas on the earth may 

 have been the scenes of the foundering of batholithic roofs. Such foundering 

 must have been more likely to occur in the earlier pre-Cambrian time. It has 

 not often occurred in post-Archean time, probably because the available 

 heat in abyssal injections of this period has been too small to thin, and thus 

 weaken, batholithic roofs sufficiently. 



Lavas which reach the surface through the perforation of roofs by blow- 

 piping gases (either juvenile or resurgent) are again of great variety of com- 

 position. Because of the conditions special to these ' central eruptions ' at the 

 earth's surface, the petrographic variety of rock types is here greater than it is 

 in fissure eruptions or in intruded bodies. The cause of this contrast is chiefly 

 found in the larger chance for magmatic differentiation in the main vents of 

 central eruptions. 



Genetic Classification of Magmas. 



The magmas from which the igneous rock3 have crystallized may be gene- 

 tically classified as in the following list, which gives under each head a num- 

 ber of examples or rocks corresponding to the magma types. 



1. Primary basaltic magma (primary in the sense that it has persisted in the 



molten or potentially molten state since the time of the oldest pre- 

 Cambrian greenstones). 



Representative rocks: basaltic lava, gabbro, diabase, some basic porphy- 

 rites, etc. 



2. Primary granitic magma of the earth's acid crust. 



Representative rocks'. Perhaps none crystallized directly from this pre- 

 Keewatin magma; indirectly represented in the acid granites of the 

 pre-Cambrian batholiths. 



3. Direct magmatic differentiates of primary basalt. 



Representative rocks : augite andesite, certain peridotites, anorthosite. 



4. Syntectic magmas. 



A. Syntectics chiefly composed of primary basalt and primary acid earth- 



shell. 

 Representative rocks: diorites, certain porphyrites, etc. 



B. Syntectics chiefly composed of primary basalt and sediments. 



Representative rocks: some hybrid types. 



C. Syntectics composed of primary basalt and essential amounts of both 



acid shell and sediments. 

 Representative rocks: some hybrid types. 



5. Magmatic differentiates of syntectics of Class A. 



Representative rocks: most granites; many aplites and lamprophyres. 



