REVIEWS 761 
which is expressed both on heating and cooling and a change in the heat 
capacity. 
Quartz formed at the low temperature is hexagonal and trapezohedral- 
tetratohedral, while that formed at the high temperature is probably 
hexagonal and trapezohedral-hemihedral. It is found that these crystallo- 
graphic relations are expressed in the circular polarization of the two 
varieties. Intergrowths of right- and left-handed quartzes are more fre- 
quent and more regular in boundary lines in the quartz formed at low 
temperatures and the etched figures show more regular and more sharply 
defined twinning. Quartz formed at the higher temperatures and subse- 
quently cooled may show the effects of inversion by shattering or latent 
weakness inherent in such crystals may be disclosed by etching in hydro- 
fluoric acid. Large clear quartzes free from cracks have probably never 
reached the inversion temperature. 
Quartzes from veins and geodes and certain vein pegmatites are in 
general clear and free from intricate fracture-cracks and show regular 
intergrowths of right- and left-handed quartzes; they are frequently twinned 
and the outline of the twinned areas is usually regular. The quartzes 
from graphic and granite pegmatites, granites, and porphyries are smaller 
in size, frequently fractured and cracked. They rarely show intergrowths 
of right- and left-handed individuals and the outlines of such intergrowths 
may or may not be regular. The observed characteristics of the first 
group of quartzes are those deduced theoretically for low-temperature 
quartzes, while the features of the second group are those deduced theoreti- 
cally for quartzes formed above 575°. This places the temperature of 
final solidification of an intrusive granite mass above 575°. 
It is hoped that these investigations will be extended to the quartz 
formed by contact metamorphic processes. As pointed out by Lindgren 
these deposits without doubt were formed above the critical temperature 
of water. By the use of the quartz thermometer on carefully selected 
material the precipitation point of many minerals of the contact zones 
could be approximated much more closely than has yet been done. 
Tridymite, it is pointed out, forms at 800°, beyond which the high-tem- 
perature quartz is unstable. Definite data concerning the behavior of 
SiO, through a wide range of temperatures and at ordinary pressures are 
thus supplied. Perhaps the pressures do not greatly affect the nature of 
crystallization though they do affect the solubility of quartz. In the great 
majority of the rhyolites and quartz-porphyries of Montana, Nevada, 
and Colorado, it is, in the experience of the reviewer, the most conspicuous 
of the phenocrysts. Nearly everywhere it is resorbed. The phenocrysts 
