208 DR. THOMAS STERRY HUNT ON THE 
hydrous silicates, the view which supposes the olivine or the enstatite to be simply an 
instance of the crystallization of an anhydrous silicate in the midst ofan amorphous hydrous 
silicate, is more consonant with the hypothesis of the aqueous origin of serpentine-rocks. 
It is well known that Scheerer, from his studies of the associated olivine and serpentine 
of Snarum, was led to reject the notion of the derivation of this serpentine from a pre- 
viously-formed olivine, and to maintain a simultaneous formation of the anhydrous and 
the hydrous silicates.* 
A somewhat analogous case is presented in the occurrence of grains of anhy- 
drous alumina or corundum found in the earthy and amorphous aluminous hydrate, 
bauxite, which forms beds in uncrystalline cenozoic rocks. | The notion which has been 
advanced that the bauxite has come from the hydration of previously-formed beds of 
corundum is obviously untenable, and we must regard this anhydrous alumina as formed 
by crystallization in the midst of the uncrystalline mass of hydrated alumina. De Senar- 
mont, in the decomposition of aqueous solutions of chlorid of aluminum, at 250° C. 
observed a simultaneous production of anhydrous alumina in the form of corundum, 
and of hydrous alumina as diaspore, both crystallized. £ 
§ 110. The late studies of Arno Behr throw further light on the association of 
hydrous and anhydrous species. He has found that solutions of dextrose, within very 
narrow limits of temperature and concentration, yield crystals either of hydrated or 
anhydrous dextrose, and that under certain conditions we can obtain an admixture of the 
two, as the result of simultaneous crystallization. § 
A illustration of the influence of small variations in composition on the result of a 
chemical process under conditions otherwise similar, is afforded by the recent experiments 
of Friedel and Sarrasin on the artificial production of albite inthe wet way. When a solu- 
tion of silicate of soda mixed with silicate of alumina in the proportions required to form 
the soda-feldspar, was heated in close vessels to from 400° to 500° C., no albite was formed, 
but crystals of the hydrated double silicate, analcime; silica, soda, and some alumina 
remaining in solution. When, however, an excess of the alkaline silicate was employed, 
the whole of the silicate of alumina was converted into a crystallized anhydrous compound, 
which was albite. || 
§ 119. Much obscurity still surrounds the question of the conversion of olivine into 
serpentine. In the first place, it is to be remembered that the process is one which does 
not, under ordinary circumstances, take place at or near the surface of the earth, since 
olivine-rocks, whether exotic masses or indigenous crystalline schists, are often met with, 
presenting no evidence of such change. This is well seen near Montreal, where the hills 
of olivine-dolerite, demonstrably of pre-Silurian age, as well as fragments of the same 
rock imbedded in Silurian conglomerates, alike contain only unaltered anhydrous olivine. 
This mineral, on exposed surfaces, is subject to a subaérial decay, analogous to that 


* Scheerer, Pogg. Annalen. Ixviii., 319, and Amer, Jour. Science [2] v. 389, vi., 201, also xvi., 217. 
+ Deville, An. de Ch. et de Phys. [3J Ixi., 309,and Hunt, Origin of Some Magnesian and Aluminous Rocks. 
{ Comptes Rendus de l’Acad. des Sciences, 1856, xxxii., 762. 
Amer. Jour. Sci., 1861 [2] xxxii., 281, also, Chem. and Geol. Essays, p. 326. 
4 For these facts I am indebted to a private communication from Dr. Behr. See also, his paper in Jours 
Amer. Chem. Soc., in 1882., vol. iv., p. 11. 
|| Comptes Rendus de l’Acad. des Sciences, July 30, 1883. 

