Parr I. Scr. iv. §2.) METAMORPHISM. 301 
what happens within the crust of the earth, Daubrée’s experiments 
possess a high interest and suggestiveness in regard to the internal 
rearrangements and new structures which water may superinduce upon 
rocks. Hermetically sealed glass tubes containing scarcely one-third of 
their weight of water and exposed for several days to a temperature 
below an incipient red heat, showed not only a thorough transformation 
of structure into a white, porous, kaolin-like substance, encrusted with 
innumerable bipyramidal crystals of quartz like those of the drusy 
cavities of rocks, but had acquired a very distinct fibrous and even 
an eminently schistose structure. The glass was found to split 
readily into concentric lamine arranged in a general way parallel 
to the original surfaces of the tube, and so thin that ten of 
them could be counted in a breadth of a single millimetre. Hven 
where the glass though attacked retained its vitreous character, 
these fine zones appeared like the lines of an agate. The whole 
structure recalled that of some schistose and crystalline rocks. 
Treated with acid the altered glass crumbled and permitted the 
isolation of certain nearly opaque globules and of some minute 
transparent infusible acicular crystals or microliths, sometimes 
grouped in bundles and reacting on polarized light. Reduced to thin 
slices and examined under the microscope with a magnifying power 
of 300 diameters, the altered glass presented: Ist, Spherulites, 3, of 
a millimetre in radius, nearly opaque, yellowish, bristling with points 
which perhaps belong to a kind of crystallization, and with an 
internal radiating fibrous structure (these resist the action of con- 
centrated hydrochloric acid, whence they cannot be a zeolite, but 
may be a substance like chalcedony); 2nd, innumerable colourless 
acicular microliths, with a frequently stellate, more rarely solitary 
distribution, resisting the action of acid like quartz or an anhydrous 
silicate; 3rd, dark green crystals of pyroxene (diopside). Daubrée 
satisfied himself that these enclosures did not pre-exist in the glass, 
but were developed in it during the process of alteration.* 
Scheerer, Elie de Beaumont, and Daubrée have shown how the 
presence of a comparatively small quantity of water in eruptive 
igneous rocks may have contributed to suspend their solidification, 
and to promote the crystallization of their silicates at temperatures 
considerably below the point of fusion and in a succession different 
from their relative order of fusibility. In this way the solidification 
of quartz in granite after the crystallization of the silicates, which 
would be unintelligible on the supposition of mere dry fusion, 
becomes explicable, likewise the enclosure of highly fusible augite in 
the nearly infusible leucite of some Italian lavas. The water may be 
1 Géol. Experim. p. 158 et seq. The production of crystals and microliths in the 
devitrification of glass at comparatively low temperatures by the action of water is of 
great interest. The first observer who described the phenomenon appears to have 
been Brewster, who, in the second decade of this century, studied the effect upon 
polarized light of glass decomposed by ordinary meteoric action. (Phil. Trans. 1814. 
Trans. Roy. Soc. Edin. xxii. (1860) p. 607. See on the weathering of rocks, Part II. of 
this Book, p. 333.) 
