300 DYNAMICAL GEOLOGY. [Book TIL. 
silica, alkaliferous silicates, and iron oxide even at ordinary tem- 
peratures. The mere presence, therefore, of water within the pores 
of subterranean rocks cannot but give rise to changes in the com- 
position of these rocks. Some of the more soluble materials must be 
dissolved, and, as the water evaporates, must be redeposited in a new 
form.’ 
This power increased by heat.—The chemical action of water 
is increased by heat, which may be either the earth’s original heat 
or that which arises from internal crushing of the crust. Mere 
descent from the surface into successive isogeotherms raises the 
temperature of permeating water until it may greatly exceed the 
boiling point. But a high temperature is not necessary for many 
important mineral rearrangements. Daubrée has proved that very 
moderate heat, not more that 50° C. (122° Fahr.) has sufficed for 
the production of zeolites in Roman bricks by the mineral waters of 
Plombiéres.? He has experimentally demonstrated the vast increase 
of chemical activity of water with augmentation of its temperature, 
by exposing a glass tube containing about half its weight of water to 
a temperature of about 400° C. At the end of a week he found the 
tube so entirely changed into a white, opaque, powdery mass as to 
present not the least resemblance to glass. The remaining water 
was highly charged with an alkaline silicate containing 63 per 
cent. of soda and 37 per cent. of silica, with traces of potash and — 
lime. The white solid substance was ascertained to be composed 
almost entirely of crystalline materials, partly in the form of 
minute perfectly limpid bipyramidal crystals of quartz, but chiefly 
of very small acicular prisms of wollastonite. It was found, more- 
over, that the portion of the tube which had not been directly in 
contact with the water was as much altered as the rest, whence it 
was inferred that at these high temperatures and pressures the 
vapour of water acts chemically like the water itself. 
Co-operation of pressure.—The effect of pressure must be 
recognized as most important in enabling water, especially when 
heated, to dissolve and retain in solution a larger quantity of mineral 
matter than it could otherwise do.? In Daubrée’s experiments just 
cited, the tubes were hermetically sealed and secured against fracture, 
so that the pressure of the greatly superheated vapour had full 
effect. By this means, with alkaline water, he not only produced 
the two minerals above mentioned, but also felspar and diopside, 
The enormous pressures under which many crystalline rocks have 
solidified is indicated by the liquid carbon dioxide in the vesicles of 
their crystals. 
Experiments in metamorphism.— Besides showing the solvent 
power of super-heated water and vapour upon glass in illustration of 
1 See further on this subject, Part II. p. 353. 
2 Geologie Lxpérimentale, p. 462. 
* Sorby has shown that the solubility of all salts which exhibit contraction in gsolu- 
tion is remarkably increased by pressure. Proc. Roy. Soc. (1862-3), p. 340. 
