GEOPHYSICAL LABORATORY.* 



Arthur L. Day, Director. 

 SEGREGATION IN IGNEOUS ROCKS UNDER THE ACTION OF GRAVITY. 



Of the causes which may contribute to the segregation of particular 

 groups of minerals (rocks) out of a great mass of molten magma, one 

 of the most important is the condition of chemical equilibrium in con- 

 sequence of which certain minerals or groups of minerals separate out 

 through supersaturation at particular temperatures and pressures. Of 

 these, many illustrations have been cited in these reports from year to 

 year. Inunediately consequent upon this separation there arises the 

 question of the effect of gravity upon the separated portion; whether 

 the viscosity of fluid rocks or the temperature gradient upward or 

 downward or some other counteracting force will enter to prevent the 

 settling out of the nascent crystals, especially when the density differ- 

 ences are small. 



Some very patient consideration has been given both to the field 

 and laboratory evidence upon this question during the past year, from 

 which most interesting conclusions have resulted which will certainly 

 help to clear up another of the factors which influence the segregation 

 of the rocks. The field facts are essentially these: 



It is known to all that from time to time in various parts of the earth 

 molten rock or lava is poured out upon its surface in greater or less 

 quantity. Perhaps not so generally known is the fact that the molten 

 material at one locality may be quite different in character from that at 

 another, and that even at the same locality the lands of lava may differ 

 considerably in different eruptions. The masses which have been 

 thrown out at Lassen Peak, California, for example, are entirely differ- 

 ent in character from the lava which is boiling in the lake at Kilauea, 

 Hawaii, and some of the material of earlier eruptions in Hawaii differs 

 from both of these. 



But the geologist does not confine himself to the study of surface 

 lavas alone in seeking a solution of these problems. The molten rock 

 does not all pour out on the surface. Some of it remains in the feeding- 

 pipe, some may spread out in intercalated sheets in the surrounding 

 rocks, and some may collect in a great subterranean pool. Many such 

 subterranean masses of igneous or once-molten rock are now exposed 

 at the surface, the former cover having been worn away by the ceaseless 

 action of rivers, winds, frost, and the whole array of disintegrating 

 agents. Between different bodies of this kind, and indeed in different 

 parts of the same body, the geologist finds the same kind of variation 

 as he does between different lava-flows. They are in fact the source 

 of the lavas, and the problem of the diversity of lavas becomes the 



*Situated in Washington, D, C. 



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