G. F. Becker—Teature of Massive Pocks. 57 
become still less fluid if it were subjected to several times this 
pressure. Such material at a distance below the surface of five 
or more miles, would lose so much of its plasticity that it may be 
doubted whether it could furnish the material for an ordinary vol- 
canic eruption. Finally, if the influence of pressure on the 
fusibility were such as to make a notable difference in the fluid- 
ity of a column of lava 10,000 feet in height, the melting point 
of crystals which had separated out five miles below the sur- 
face would be so much reduced by the relief of pressure 
attendant upon eruption that they would completely fuse. The 
preceding paragraphs show that this is not the case with those 
minerals which form the larger individuals in porphyries. But 
hornblende, pyroxene, mica, plagioclase and orthoclase all play 
this part in the porpbyries, and they are also the chief mineral 
constituents of the granular.rocks. The supposition that a col- 
umn of lava 10,000 feet high of uniform and representative 
composition, which is fluid near the top, can become pasty near 
the base through mere pressure thus conflicts with well-ascer- 
tained facts. It is easy to see that if the opposite law of fus- 
ibility prevailed, or if the melting point rose as the pressure 
diminished, there could be no tendency to the disappearance of 
porphyries with increasing depth. 
On the other hand the diorite of Mt. Davidson and many 
granites are known to overlie sedimentary rocks later than the 
Archeean and must, therefore, have solidified within a moderate 
distance from the surface; so that while no amount of pressure 
is known to be sufficient to prevent the formation of porphyry, 
granular rocks may form at very moderate pressures. 
The foregoing considerations seem to me to justify the fol- 
lowing conclusions: 
The relation between holocrystalline porphyries and granular 
rocks is of very different character from that which exists be- 
tween glassy rocks and those which are completely devitrified. 
Porphyries may form at any depth and no matter how slowly 
the temperature of the magma may sink. Granular rocks, 
except in a just conceivable limiting case which must be ex- 
ceedingly rare, can never have been thoroughly fluid or homo- 
geneous, but have often consolidated at pressures extremely 
moderate compared with those at which it is certain that por- 
phyries would form. When granular and porphyritic forms of 
a rock are associated, as at Mt. Davidson, the cause of differ- 
ence in texture is usually a variation in chemical composition, 
and the temperature to which the rock has been subjected 
must have been sufficient to melt portions of the mass but not 
the whole. The indications are that granular rocks have been 
formed at a lower temperature than porphyries of exactly the 
