PLUTONIC ACTION, 
585 
veyed by the Romans to baths through long conduits or 
aqueducts. The foundations of some of their works consist¬ 
ed of a bed of concrete made of lime, fragments of brick, and 
sandstone. Through this and other masonry the hot waters 
have been percolating for centuries, and have given rise to 
various zeolites^—apophyllite and chabazite among others; 
also to calcareous spar, arragonite, and fluor spar, together 
with siliceous minerals, such as opal—all found in the inter¬ 
spaces of the bricks and mortar, or constituting part of their 
re-arranged materials. The quantity of heat brought into 
action in this instance in the course of 2000 years has, no 
doubt, been enormous, but the intensity of it developed at 
any one moment has been always inconsiderable. 
From these facts and from the experiments and observa¬ 
tions of Senarmont, Daubree, Delesse, Scheerer, Sorby, Sterry 
Hunt, and others, we are led to infer that when in the bowels 
of the earth there are large volumes of matter containing wa¬ 
ter and various acids intensely heated under enormous press¬ 
ure, these subterranean fluid masses will gradually part with 
their heat by the escape of steam and various gases through 
fissures, producing hot springs; or by the passage of the 
same through the pores of the overlying and injected rocks. 
Even the most compact rocks may be regarded, before they 
have been exposed to the air and dried, in the light of sponges 
filled with water. According to the experiments of Henry, 
water, under a hydrostatic pressure of 96 feet, will absorb 
three times as much carbonic acid gas as it can under the 
ordinary pressure of the atmosphere. There are other gases, 
as well as the carbonic acid, which water absorbs, and more 
rapidly in proportion to the amount of pressure. Although 
the gaseous matter first absorbed would soon be condensed, 
and part with its heat, yet the continual arrival of fresh sup¬ 
plies from below might, in the course of ages, cause the tem¬ 
perature of the water, and with it that of the containing 
rock, to be materially raised; the water acts not only as a 
vehicle of heat, but also by its affinity for various silicates, 
which, when some of the materials of the invaded rocks are 
decomposed, form quartz, feldspar, mica, and other minerals. 
As fo? quartz, it can be produced under the influence of heat 
by water holding alkaline silicates in solution, as in the case 
of the Plombieres springs. The quantity of water required, 
according to Daubree, to produce great transformations in 
the mineral structure of rocks, is very small. As to the 
heat required, silicates may be produced in the moist way 
at about incipient red heat, whereas to form the same in the 
dry way would require a much higher temperature. 
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