Col. McMahon—Granite of the Himalayas. 65 
only, on pressure; and, in the case of the Himalayan gneissose- 
granite, pressure must have varied considerably from point to point 
within the area of eruption. 
The high probability that the rising granite varied in its con- 
sistency in different portions of its mass at the time of intrusion must 
not be lost sight of in dealing with a rock that occurs within an area 
that extends not for hundreds of feet, or hundreds of yards even, but 
for hundreds of miles. 
If other causes besides the absence of perfect homogeneity at the 
time of intrusion are wanted to account for the variation of structure 
to be observed in the rock, the following may be mentioned :—At the 
points where shearing was most severe, secondary heat was probably 
developed, and the resulting chemical and mineralogical action may 
have been considerable. Then, again, we cannot suppose that the 
earth-movements that produced the contortion, overfolding, and 
faulting of the strata were sudden and explosive in their character; 
doubtless they were due to long-sustained compression, and the 
movements that resulted from this compression were repeated— 
possibly with considerable rests, or intervals between—during long 
periods of time. Hach of these movements probably left its mark 
upon the rocks, and who is now to discriminate between the effects 
of the shear, traction, and pressure that accompanied the actual act 
of intrusion, and the shear and pressure caused by the earth-move- 
ments that must, in many cases, have followed the act of injection 
and have sheared and nipped the gradually cooling granite with 
a pressure that not only varied locally from place to place, but was 
applied again and again during successive stages of consolidation ? 
When I penned the article which appeared in your May, 1887, 
Number, the object I had chiefly in view was to show that the foliation 
of the granite of the N.W. Himalayas was due to pressure acting on 
an imperfectly consolidated intrusive rock prior to its complete con- 
solidation, and that it was not due to pressure-metamorphism exerted 
on a solid and cooled rock after it had attained a consolidated and 
crystalline condition. In short, that it was an incident in the 
history of the eruption, and was not, like cleavage, due to pressure 
exerted on a solid rock. The theory of pressure-metamorphism 
differs materially from the explanation I have advocated to account 
for the foliation of the granite of the N.W. Himalayas. The 
former is not concerned with a phase of the consolidation of an 
eruptive rock. Pressure-metamorphism may come into action 
whole geological ages after the last stage of the intrusion of an 
igneous rock has come to an end; and may operate on rocks of 
purely sedimentary origin. Whether or not pressure-metamorphism 
applied to solid rocks is capable of producing all the results alleged 
by the extreme advocates of the theory, is a question foreign to the 
present inquiry. All that I have contended for is, that, in the 
regions embraced by my papers, pressure applied to the granite 
after its complete consolidation was not the cause of its foliation, 
but rather pressure applied whilst it was yet in a more or less 
plastic condition. 
DECADE IlI.—VOL. V.—NO. II. b) 
