G. F. Becker 
Texture of Massive Locks. 53 
a half in length and proportionately thick, while I found one 
specimen four inches in length and about three-quarters of an 
inch in thickness. It seems unnecessary to insist that crystals 
of such a size cannot have formed after ejection. The ground- 
mass of this andesite did not differ essentially from that of an 
ordinary hornblende andesite. 
Suppose that this rock had remained in its original position 
at least five miles below the surface and under the pressure of 
some 80,000 pounds per square inch until, by gradual refriger- 
ation, the entire mass had become solid. There would probably’ 
have been no residual glass, or the mass would have become 
holoerystalline. But, as the interstices between the hornblendes 
gradually filled with other minerals, and after a certain portion 
of the entire mass had become solid, the growing crystals must 
have been crippled by mutual interference and the opposition 
of the already completed crystals. In short it is not possible 
that a more or less granular groundmass should have been en- 
tirely absent and, even if this rock had covled at an,impercep- 
tible rate and under a stupendous pressure, it would still have 
yielded a porphyry and not a rock of granitoid structure. 
Hvidence of this kind could be indefinitely adduced and it is 
in the hands of every geologist. It appears to me to lead inev- 
itably to the conclusion that porphyries may be formed from 
homogeneous fluid magmas at pressure however great, and at 
temperatures which sink at never so small a rate. If so, the 
formation of rocks of the structural type of granite from homo- 
geneous fluid magmas must represent only extreme and highly 
exceptional cases of a curious neutral chemical equilibrium. 
Conditions the very opposite of those attending the consoli- 
dation of fluid eruptive magmas are present in sedimentary 
rocks undergoing crystallization as a feature of metamor- 
phism. The chemical composition of a sedimentary rock usu- 
ally varies from millimeter to millimeter. If it is permeated 
by a chemically active fluid, the reaction which will liberate 
heat most rapidly varies with the locality as rapidly as the 
composition. Hence innumerable centers of a whole range of 
reactions are simultaneously established, a number of minerals 
equal to that of the reactions begins to crystallize at once, and 
no one mineral has, asa rule, an opportunity to exhaust the 
materials of which it is built up before it is interfered with by 
others. Thus among metamorphic crystalline rocks a granular 
structure should be the rule and a porphyritic structure should 
represent rare exceptions. This is ofcourse true. Thecommon- 
est partial exception is in the case of garnet and is due to the 
power which garnet possesses to an unusual degree of including 
solid foreign material. There are cases however which I have 
carefully studied, in which a metamorphic mass for a few cubic 
