20 T. STERRY HUNT ON THE GENETIC 
heavier chrysolite by a process like that imagined by Durocher. It will be noticed that 
the insoluble and non-chrysolitic portion separated from the Montarville rock (V) is near 
in composition to an ordinary dolerite, or to the normal basic types of Bunsen and Duro- 
cher. We may conjecture that dolerites of average composition are, perhaps, themselves 
products separated by eliquation from a more chrysolitic aggregate. 
§ 32. The segregation of groups of crystals which takes place in the devitrification of 
glasses shows, within narrow limits, the process of differentiation through crystallization 
in a homogeneous mass. The operation of this process on a larger scale, giving rise to 
remarkable mineralogical differences, is well shown in the careful studies by Fouqué, in 
1873, on the recent eruptive rocks from Santorin. The ordinary type of these lavas 
examined by him was a vitreous mass enclosing crystals of feldspars, with pyroxene, 
chrysolite, and magnetite. The feldspar was chiefly labradorite, but its association with 
crystals of albite, and with some anorthite, was established. Druses in this same rock 
were, however, filled with anorthite, associated with a pyroxene and a chrysolite, both 
differing from those contained in the paste in being less dense, and in containing less 
ferrous oxyd. In an obsidian-like rock from the same region were rounded masses, some- 
times a metre in diameter, gray in color, and made up of crystalline anorthite, with 
pyroxene, chrysolite, titanite, and magnetite, with very little paste. The small portions 
of alumina found in the analyses of these pyroxenes were apparently, according to Fouqué, 
derived from adherent anorthite, but another variety of pyroxene, seemingly very pure, 
and freed from anorthite, contained 12.4 per cent of alumina, which he regards as an 
integral part of the mineral,—a true aluminous pyroxene. Fouqué made use, in these 
investigations, of concentrated fluorhydric acid which readily attacks the coarsely powdered 
rock, dissolving alike the vitreous paste, albite, labradorite, and anorthite, but leaving 
behind the pyroxene and chrysolite, which, like amphibole, are but slightly attacked by 
the acid, or, like staurolite and zircon, resist its action. 
§ 33. Durocher, in his statement of the hypothesis of eliquation as applied to eruptive 
rocks, of which this process of segregation just noticed is but an illustration, raises, in 
connection with the question of differentiation, another not less important. He concludes 
from his comparative studies-that, “in the long course of the ages which divide the 
Primary and the Tertiary periods from each other,’ there have been changes “in the 
composition of the fluid mass which nourished the eruptions”; and, moreover, that in 
the case of the acidic layer—the source of the granitic and trachytic rocks— “there was a 
diminution of eight or nine hundredths in the proportion of silica, and of one-fifth in the 
potash, while the proportions of lime and iron-oxyd were almost doubled, and that of the 
soda tripled. Similar changes, according to him, have taken place in the basic layer, 
represented by dolerites, basalts, melaphyres, from the comparative study of which he 
* Fouqué, Nouveau procédé pour l'analyse médiate, et son application aux layes de la dernière eruption de 
Santorin; in abstract, Comptes Rendus de l’Académie des Sciences, June, 1873, Ixxvi. 1181. Also, in extenso, Mem. 
des Savants Etrangers, de l’Acad. des Sciencies, xxii. no. 11. For farther details of this use of fluorhydric acid, see 
Fouqué and Michel Lévy, Minéralogie Micrographique, p. 116. Crystals of zircon from different localities, accord- 
ing to the late observations of Ed. Linnemann, when exposed for ten days to the vapors of fluorhydric acid, crumble 
to a white powder, which is not attacked by fluorhydric acid nor by aqua regia, and is pure silicate of zirconia, 
equal to 93 and 94 per cent. of the crystals. The matters attacked are silicates of various bases, including alkalies, 
lime, magnesia, iron, zinc, and alumina. (Sitz. Berichte Kais. Acad. Wissenschaft, 11, 1885, in Chem. News, 
Nov. 6, 1885.) 
