648 PROCEEDINGS OF THE KEW YORK MEETING 



by defluite, if not exact, luimerical measurements, showing that the grain 

 in some cases did vary as it would if the rate of cooling were the dominant 

 factor. It must be remembered, too, that it is more than probable that most 

 igneous magmas contain catalyzers — gases of one kind or another that promote 

 liquidity and restrain viscosity — and that, as regards the later formed minerals, 

 the earlier act as centers or nuclei of crystallization, notably orthite in 

 granite, promoting crystallization and preventing undercooling, thus making 

 the grain more responsive to the rate of cooling. Indeed, the jars recorded 

 by seismographs may have some effect. 



To give more concrete expression to our conclusions, let us insert some 

 hypothetical figures : 



Suppose the hornblendic country rock was at first at 100 + degrees centi- 

 grade (the close welded contacts suggest that it was not wet). The tempera- 

 ture of formation of the aplites was probably not above SOD degi-ees centigrade 

 (the inversion point of quartz to tridymite, according to Day and Allen, is 

 about 760 degrees at ordinary pressures). If so. the initial temperature of 

 tlie aplite-formiug magma was a little less than 1,500 degrees (2 X 800 — 100) 

 + 100 : but b,y the time the coarsest pegmatites of this region formed the coun- 

 try rock temperature may have risen to over 700 degrees and the magma 

 temperature fallen to less than 900 degrees, if we suppose the grain to be 50 

 times greater in the pegmatite than in the aplite. 



This is merely an illustration, for I am inclined to think that the inter- 

 change of vapor pressure, or gases, is quite as important as the interchange 

 of heat, and the growth of quartz crystals from the margin of the pegmatites 

 is especially due to it. 



The next paper was 



OPHITIC TEXTURE 

 BY A. C. LAjSTE 



[Ahstract] 



Ophitic texture is not a mere synonym of the diabasic. but a variety of 

 the poikilitic* made by the inclosure of tabular, or lath-shaped, feldspar in 

 augite. by which a mottled appearance is often produced, like that of some 

 ophidians (figure 1, plate 70). This mottling, when coarse enough, appears: 



(1) On smooth, dull, or matte surfaces, like those of pebbles or diamond 

 drill cores (figure 2, plate 70). 



(2) On fresh fracture very faintly, but in the sunlight the reflection on large, 

 interrupted cleavage faces of the augite produces the effect called by Pumpelly 

 "luster-mottling." 



(.3) On joints and rifts the pattern is brought out in a variety of color 

 effects, according to the type of weathering. 



(4) On weathered surfaces that are also exposed to erosion a lumpy, warty, 

 or pock-marked appearance is produced, which gave rise to the name "varioloid 

 greenstone," as applied to ophitic melaphyres (plate 71). 



♦Geological Survey of Michigan, vol. vi, part 1, pp. 51, 227; Journal of Geology, 

 1906, p. 705. 



