302 The America7i Geologist. May, i9«> 
crust, but a great volume of the Cambrian strata. Hence the 
Cambrian strata near the contact were highly heated and in- 
capable of chilling the magma. Finally the heat invasion 
reached its utmost limit and began to retreat; and, as with the 
tide, the recession must have been at first extremely slow. 
But it was precisely during this period of slowest and most 
gradual surface cooling that the contact zone of quartz por- 
phyry, covering many square miles of the batholith, must have 
been formed. 
According to the best statements of the accepted theory 
which I have seen we must conceive that the magma as a 
whole contained crystals of the first consolidation, due, in 
part, to pressure ; and that while in the superficial portion of 
the batholith, through the development of a groundmass by a 
comparatively sudden decrease of temperature or increase of 
pressure, these crystals retain their idiomorphic character, in 
the central portion of the batholith they become allotrio- 
morphic by continuing to grow until the magma is exhausted. 
In whatever form it is stated, this theor}' postulates as its 
most essential and vital feature not merely a more rapid 
change (decrease of temperature or water or increase of pres- 
sure, or all three) in the superficial than in the deep-seated por- 
tions of the batholith, which must be granted, but at some 
point in the process of crystallization of the superficial magma 
a more or less abrupt increase in the rate of change. This in- 
crease is provided for volcanic rocks by their effusion; and 
for intrusive or dike rocks by their transfer from lower to 
higher levels in the crust, although, as Pirsson has shown, the 
distribution of phenocrysts in dikes is not always or usually 
in harmony with this idea. But for truly plutonic masses the 
case is different; and I was first led to question the sufficiency 
of this theory as a general explanation of phenocrysts and por- 
phyritic texture by the difficulty of conceiving the possibility 
of such a break in the physical conditions for even the super- 
ficial part of a batholith. 
That the growth of the phenocr}-sts has been arrested by 
the relatively sudden solidification of the magma residuum to 
form the groundmass is, to my mind, unquestionable. The 
real question is as to the cause of this change or crisis in the 
history of the rock. Being unable to recognize the possibility 
