298 _-«-DYNAMICAL’GEOLOGY. —[Boog III. 
‘of heat modified by compression, he obtained by sublimation — 
“transparent and well-defined crystals,” lining the unoccupied portion 
of a hermetically-sealed iron tube, in which he had placed and 
exposed to a high temperature some fragments of limestone.’ 
Numerous experiments have been made by Delesse, Daubrée, and 
others, in the production of minerals by sublimation. Thus, many of 
the metallic sulphides found in mineral veins have been produced by 
exposing to a comparatively low temperature (between that of 
boiling water and a dull-red heat) tubes containing metallic chlorides 
and sulphide of hydrogen. By varying the materials employed, 
corundum, quartz, apatite, and other minerals have been obtained, 
It is not difficult, therefore, to understand how, in the crevices of 
lava-streams and volcanic cones, as well as in mineral veins, sulphides 
and oxides of iron and other minerals may have been formed by the 
ascent of heated vapours. Superheated steam is endowed with a 
remarkable power of dissolving that intractable substance, silica; 
artificially heated to the temperature of the melting. point of cast- 
iron, it rapidly attacks silica, and deposits the mineral in snow- 
white crystals as it cools. Sublimation, however, can hardly be 
conceived as having operated in the formation of rocks, save here 
and there in the infilling of open fissures. 
§2. Influence of Heated Water—Metamorphism. 
In the geological contest fought at the beginning of the century 
between the Neptuunists and the Plutonists, the two great battle-cries 
were, on the one side, Water, on the other, Fire. ‘The progress of 
science since that time has shown that each of the parties had some 
truth on its side, and had seized one aspect of the problems touching 
the origin of rocks. If subterranean heat has played a large part in the 
construction of the materials of the earth’s crust, water, on the other 
hand, has performed a hardly less important share of the task. They 
have often co-operated together, and in such a way that the result 
must be regarded as their joint achievement, wherein the respective 
share of each can hardly be exactly apportioned. In Part II. of this. 
Book the chemical operation of infiltrating water at ordinary tem- 
peratures at the surface and among rocks at limited depths is 
described. We are here concerned mainly with the work done by 
water when within the influence of subterranean heat. 
Presence of water in all rocks.—By numerous observations it 
has been proved that all rocks within the accessible portion of the 
earth’s crust contain interstitial water, or, as it is sometimes called, 
quarry-water (eau de carriere). This is not chemically combined 
with their mineral constituents, nor hermetically sealed up in vesicles, © 
but is merely retained in their pores. Most of it evaporates when 
the stone is taken out of the parent rock and freely exposed to the 
atmosphere, The absorbent powers of rocks vary greatly, and chiefly 
1 Trans. Roy. Soc. Edin. vi. p. 110, 
