206 abstracts: petrology 



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A discussion of the method whereby accumulation of plagioclase 

 crystals might take place leads to the conclusion that the most prom- 

 ising method is the separation by gravity of the femic constituents 

 from gabbroic magma while the plagioclase crystals, which are calcic 

 bytownite, remain practically suspended. Then, at a later stage, when 

 the liquid has become distinctly lighter, having attained diorite-syenite 

 composition, the plagioclase crystals, which are now labradorite, 

 accumulate by sinking and give masses of anorthosite, at the same 

 time leaving the liquid out of which they settle of a syenitic or granitic 

 composition. 



Some of the consequences of this manner of origin of anorthosite are 

 discussed. 



A consideration of anorthosites with special reference to the Adiron- 

 dack and Morin areas gives some reason for believing that anorthosites 

 show the requisite characters. For the Adirondack area especially, 

 evidence is adduced favoring the possibility that there anorthosite 

 and syenite may still occupy the relative positions in which they were 

 generated by the process outlined, the Adirondack complex being 

 interpreted as a sheetlike mass with syenite above and anorthosite 

 below. 



Other monomineral rocks present essentially the same problem and 

 are restricted in their occurrence in substantially the same manner if 

 we consider especially those that approach most closely to the strictly 

 one-mineral character. All of the monomineral rocks do occur, how- 

 ever, as dikes and dike-like masses in essentially contemporaneous, 

 congeneric igneous rocks, a fact which may be interpreted as due to 

 the intrusion of a heterogeneous, partly crystalline mass. 



On the whole the inquiry gives considerable support to the belief 

 that the monomineral rocks, of which the anorthosites are perhaps the 

 most important representatives, are generated by the process of col- 

 lection of crystals under the action of gravity. 



N. L. B. 



PETROLOGY. — Adirondack intrusives. N. L. Bowen. Journ. Geol. 

 25: 509-512. Sept.-Oct., 1917. 

 A reply to criticism by Professor Gushing of certain statements 

 relative to Adirondack structure occurring in the paper abstracted 

 above. 



N. L. B. 



