50 G. EF. Becker—Texture of Massive Rocks. 
tinguishing certain lavas as quartz-basalts, just as it is used in 
other cases to characterize quartz andesite. The separation of 
the basalts into ordinary basalts and quartz basalts is nota 
purely mineralogical classification but is founded upon their 
chemical composition as well as upon their genetic relations 
expressed in their natural order of eruption—the quartz basalts 
in every case known being younger than the other basalts of 
the same region just as the quartz andesites (dacites) are 
younger than the other andesites of the same voleanic center. 
Petrol. Lab. U. 8S. Geol. Survey, Washington, D. C., Nov. 19, 1886. 
Art. VI.—The Texture of Massive Rocks; by GnHoRGE F, 
BECKER. 
AN opinion has been gaining ground among geologists for 
some years past, that the principal differences in the texture 
of massive rocks are due to differences in the rate of cooling 
and differences in the pressure under which the masses have 
consolidated. That slow cooling will convert into a partially 
crystalline mass a magma which, if rapidly chilled, would re- 
main substantially in the form of glassis known experimentally* 
and observations on volcanic flows are accordant with exper- 
iment; for it is certain that glass is often abundant at the sur- 
face of lava streams and that it is chiefly confined to moderate 
distances from the original surfaces of eruptive masses, although 
minute particles of glass may frequently be found at consider- 
able depths. It may readily be granted, that ata sufficient 
depth all trace of glass would disappear; or that an ordinary, 
homogeneous, eruptive magma may be cooled so slowly and 
under such pressures as to yield a holocrystalline product. 
Whether or not the pressure is influential in this respect is 
questionable. 
It is supposed to be a mere extension of this thesis to main- 
tain that the difference between porphyritic structure and gran- 
ular structure is due to a similar difference in the physical con- 
ditions to which the fluid magma has been subjected. The 
inference has thus been drawn, that ifthe material constituting 
a given holocrystalline porphyry had been cooled sufficiently 
slowly and under sufficient pressure, it would have assumed 
the granular structure of typical granite or diorite. 
This appears to me to be a wholly different and almost an 
unrelated proposition rather than a legitimate extension of the 
preceding. ‘The immediate origin of granular structure is the 
* The best example is perhaps “‘ basaltified ” iron blast-furnace slag, which has 
been used to a large extent for street pavements. 
