HISTORY OF CRYSTALLINE ROCKS. 25 
mass impregnated with water and not yet raised to the full temperature of solution, or 
what has been aptly termed ‘“igneo-aqueous fusion,” the more soluble portions, removed 
by percolation or by diffusion, we conceive to have constituted the liquids which in 
earlier times produced the various crenitic rocks. The fact that, as shown by Sorby, ! 
pressure augments the solvent power of water, irrespective of temperature, should not be 
lost sight of in this connection. The remarkable observations of Tilden and Shenstone 
serve to explain and to justify the view of the intervention of water in giving liquidity 
to various eruptive rocks, originally put forward by Poulett Scrope, and afterwards ably 
maintained, among others, by Scheerer and Elie de Beaumont. ? 
The conversion of colloidal magmas, whether hydrous as just described, or an- 
hydrous, as noticed in § 26, into denser crystalline species, not only involves the disengage- 
ment of heat, but as Becker has shown, its disengagement at a maximum rate, thus 
maintaining, with the temperature, the liquidity of the crystallizing magma.’ The pas- 
sage of certain dense species, as epidote, zoisite, garnet, beryl and quartz, when fused per 
se, into vitreous or crystalline forms of less specific gravity ‘ is no exception to this law of 
condensation, since the chemical and physical conditions of the fused mass are unlike those 
of the more complex magma. When such a magma, holding combined a portion of water, 
is changed into anhydrous crystalline species, this will be liberated, as is shown in the 
often observed disengagement from solidifying lavas, of aqueous vapor, sometimes with 
boric oxyd, fluorhydric aud chlorhydric acids, and various chlorids. Hence crystalline 
silicates like epidote, tourmaline, and micas, which contain these volatile elements, will 
only be generated under such conditions as prevent their liberation. 
$ 44. We have already noticed the banded structure (§ 28) which often results from 
movement in the extrusion of more or less differentiated masses of eruptive rocks, simulat- 
ing that produced by the separation from water either of mechanical sediments or of crys- 
talline deposits. It is important in this connection to distinguish between the latter two 
processes, and to insist upon the more or less concretionary character of the matters separ- 
ated from solution, often shown in the lenticular shape of beds of this character, and well 
displayed in the crystalline schists. The conditions under which these were laid down 
from water were less like those of ordinary sediments than of the accumulations of crys- 
talline matter in geodes and in veins. Many facts with regard to the banded character 
of mineral veins are familiar to geologists, and the stratiform character of such deposits 
has often been remarked in smaller vein-like masses. I have elsewhere called attention to 
the fact that crystalline masses having the relations of veinstones may assume great pro- 
portions, and that much granitic rock often regarded as eruptive is in fact of concretionary 
and endogenous origin, discussing the question at some length in 1871.° Veins of this 
kind were then described sixty feet in breadth, traversing the gneisses and mica-schists 
of the younger gneissic or Montalban series, in New England, often coarsely crystalline 

! Proc, Roy. Soc. London, xii. 538. 
? Scrope, Jour. Geol. Soc. London, xii., 326; Scheerer, Bull. Soc. Geol. de France, 1845, iv. 468; and Elie de 
Beaumont, Zbid., 1240 et seq. See farther the author’s Chem. and Geol. Essays, 188-191, and also 5, 6, for farther 
references to the literature of the subject. 
* Becker, Amer. Jour. Science, 1886, xxxi. 120. 
* A Natural System of Mineralogy, etc. Trans. Roy. Soc. Can. Vol. iii. Sec. iii. p. 36. 
5 Granites and Granitic Veinstones, Amer. Jour. Science, 1871. Chem. and Geol. Essays, pp. 191-202. 
Sec. IIT., 1886. 4. 
