ORIGIN OF AUGITE ANDESITE 403 



and 1135° C; the magnetite, largely between 1195° and 1100° C. 

 The range for phenocrystic augite is 1190-960° C, with the most 

 abundant crystallization between 1190° and 1100° C. The range for 

 labradorite is ii25°-io75° C. He observed augite phenocrysts 

 developed in molten basalt at the range, io85°-920° C; in niohen 

 limburgite at 1150°. Magnetite formed abundantly in molten basalt 

 at 1095° C. and in molten limburgite at various temperatures ranging 

 from 1170° to 1065° C. For rock-melts he records only one deter- 

 mination for olivine which "probably" crystaUized out at 1085° C. 

 in molten basalt. 



Throughout most of the period of phenocrysitc development, that 

 is, through a fall of temperature from 1200° to about 1080° C, 

 basaltic lava is still notably fluid. Other experiments by Doelter 

 have shown that strong fluidity characterizes various basic lavas at 

 the following respective temperatures : 



Etna basalt . 1010° C. 



Remagen basalt 1060 



Vesuvian lava 1080 



Limburgite 1050 



It is fair to conclude that at the temperature of 1050° C. the average 

 oHvine basalt is fluid, and at 1100° C. quite thinly fluid. At the 

 latter temperature its kinetic viscosity is probably comparable to that 

 of the Hawaiian basaltic flow which Becker has calculated to have 

 had, at the time of its emission, a viscosity about fifty times that of 

 water.' 



SINKING OF THE PHENOCRYSTS 



In lava of such relatively high fluidity the olivine, augite, and 

 magnetite crystals must slowly sink. Combining the results obtained 

 by C. Barus"" and, more recently, by J. A. Douglas,^ on studies of 

 volume changes as basic rock changes from the holocrystalline state 

 to the glassy and then to the molten condition, the present writer 

 has calculated that olivine basalt would have, at 1100° C. and at one 

 atmosphere of pressure, a specific gravity averaging about 2.74; 

 its groundmass, specific gravity ranging from 2.55 to 2.60. At the 



I G. F. Becker, Amer. Jour, of Sci., Vol. Ill, 1897, P- ^9- 

 ' Bull. No. 103., U. S. Geological Survey, 1893. 

 3 Quart. Jour. Geol., Vol. LXIII, 1907, p. 145. 



