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ORIGIN OF CRYSTALLINE ROCKS. 497 
and iron-oxyd are deposited during the evaporation of natural 
waters, including those of alkaline springs and of the Ottawa 
River. Having described the mode of occurrence of the mag- 
nesian silicate, sepiolite, in the Paris basin, and the related 
quincite, containing some iron-oxyd and disseminated in lime- 
stone, I suggested that while steatite has been derived from a 
compound like sepiolite, the source of serpentine was to be sought 
in another silicate richer in magnesia; and, moreover, that chlo- 
rite, unless the result of a subsequent reaction between clay and 
carbonate of magnesia, was directly formed by a process analogous 
to that which (according to Scheerer) has,in recent times, caused 
the deposition from waters of neolite, a hydrous alumino-magne- 
sian silicate approaching to chlorite in composition,* ‘‘ the type of 
a reaction which formerly generated beds of chlorite in the same 
way as those of sepiolite or tale.” Delesse, subsequently, in 1861, 
in his essay on Rock-Metamorphism insisted upon the sepio- 
lites or so-called magnesian marls, as probably the source of 
steatite, and suggested the derivation of serpentine, chlorite, and 
other related minerals of the crystalline schists, from deposits - 
approaching these marls in composition.y He recalled, also, the 
occurrence of chromic oxyd, a frequent accompaniment of these 
magnesian minerals, in the hydrated iron ores of the same geo- 
logical horizon with the magnesian marls in France. Delesse did 
not, however, attempt to account for the origin of these deposits 
of magnesian marls, in explanation of which I afterwards verified 
Bischof’s observations on the sparing solubility of silicate of 
magnesia, and showed that silicate of soda, or even artificial hy- 
drated silicate of lime, when added to waters containing magne- 
sian chlorid or sulphate, gives rise, by double decomposition, to a 
very insoluble magnesian silicate. { 
To explain the generation of silicio like labradorite, scapo- 
lite, garnet, and saussurite, I suggested that double aluminous 
silicates allied to the zeolites might have been formed, and subse- 
quently rendered anhydrous. The production of zeolitic minerals 
observed by Daubrée at Plombiéres and Luxeuil by the action of 
a silicated alkaline water on the masonry of ancient Roman baths, 
was appealed to by way of illustration. It had there been shown 
* Pogg. Annal., Ixxi, 288. 
t Etudes sur le Minnoch, quarto, pp. 91. Paris, 1861. 
į Amer. Jour. Sci., IT 
