94 
IDDINGS. 
They also observed the great tendency of angite and ensta- 
tite to crystallize out of a cooling magma, and remarked that 
the magma must be cooled very rapidly to prevent their 
partial separation from it.* Nepheline crystallizes rather 
easily, and the feldspars less so, while a certain group of the 
rock-making minerals fail to crystallize at all under the con¬ 
ditions of purely igneous fusion at ordinary pressures. These 
minerals are quartz, orthoclase! albite, mica, and hornblende. 
It is to be observed that the last-named minerals are those 
that characterize the more acid rocks, which it has been 
remarked more frequently occur in a glassy condition than 
the basic ones, the latter exhibiting a greater tendency to 
crystallize than the former when subjected to the same phys¬ 
ical conditions. 
On the other hand, quartz, orthoclase, and albitef have 
been produced artificially at high temperatures in the pres¬ 
ence of water vapor and other gases under high pressure, 
and mica has been produced by the simple fusion of silicates 
with fluorides .% This has led many investigators to the con¬ 
clusion that the crystallization of rocks composed of these 
minerals requires the assistance of a mineralizing agent in the 
condition of an absorbed vapor. The influence of absorbed 
vapors to promote crystallization has been observed in the 
production of artificial glasses, where it is found necessary to 
free the molten glass from gas bubbles to prevent its devitri¬ 
fication upon cooling. 
The results of experimentation are far from the solution 
of the problem before us. They have not as yet succeeded 
in producing the minerals—quartz, orthoclase, mica, and 
hornblende—in the manner in which they occur either in 
* Loc. cit., p. 49. 
| Friedel and Sarasin. Bull. Soc. Mineral. France, vol. 2, 1879, p. 158; 
ibid., vol. 3, 1880, p. 171; Comptes rendus Acad. Sci., Paris, vol. 92, 1881, 
p. 1374; ibid. ,vol. 97, 1883, pp. 290-294; K. von Chrustschoff. Am.Chemist, 
1873; Tschermak’s min. pet. Mittheil, vol. 4, 1882, p. 536. 
J Hautefeuille. Comptes rendus Acad. Sci., Paris, vol. 104,1887, p. 508 ; 
K. von Chrustschoff. Tschermak’s min. pet. Mittheil, vol. 9, 1888, p. 55. 
C. Doelter. Tschermak’s min. pet. Mittheil, vol. 10, 1889, p. 67. 
