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Parr I. Secr. iy.§1.] HYPOGENE CHANGES. 289 
‘In the present section the student is asked to consider chiefly the 
nature of the agencies by which such changes can be effected ; the. 
results achieved, in so far as they constitute part of the architecture 
or structure of the earth’s erust, will be discussed in Book IV. At 
the outset, if is evident that he can hardly hope to detect many of these. 
processes of subterranean change actually in progress and watch 
their effects. The very vastness of some of them places them beyond 
his direct reach, and he can only reason regarding them from the 
changes which he sees them to have produced. But a good number 
are of a kind which can in some measure be imitated in laboratories 
and furnaces. [tis not requisite, therefore, to speculate wholly in 
the dark on this subject. Since the early and classic researches of 
Sir James. Hall, great progress has been made in the investigation of 
hypogene processes by experiment. The conditions of nature have. 
been imitated as closely as possible, and varied in different ways,. 
with the result of giving us an increasingly clear insight into the 
physics and chemistry of subterranean geological changes. The 
following pages are chiefly devoted to an illustration of the nature of 
_ hypogene action, in so far as that can be inferred from the results of 
actual experiment. ‘The subject may be conveniently treated under: 
three heads—I. The effects of mere heat; 2. the influence of the co- 
operation of heated water; 3. the effects of pressure and contraction. 
§ 1—Effeects of Heat. 
The importance of heat among the transformations of rocks 
has been fully admitted by geologists, since it used to be the 
watchword of the Huttonian or Vuleanist school at the end of 
last century. Two sources of subterranean heat may have at 
different times and in different degrees co-operated in the pro- 
duction of hypugene ehanges—the original internal heat of the globe, 
and the heat due to the transformation of mechanical energy in 
the crumpling, fracturing, and crushing of the rocks of the crust. 
Rise of temperature by depression.—As stated above, the 
mere recession of rocks from the surface owing to superposition of 
newer deposits upon them will cause the isogeotherms, or lines of 
equal subterranean temperature, to rise—in other words, will raise the. 
temperature of the masses so withdrawn. ‘This can take place, how- 
ever, to but a limited extent unless combined with such depression 
of the crust as to admit of thick sedimentary formations. From 
the rate of increment of temperature downwards it is obvious 
that at no great depth the rocks must be at the temperature 
of boiling water, and that farther down, but still at a distanee 
which relatively to the earth’s radius is small, they must reach and 
exceed the temperatures at which they would fuse at the surface. 
Mere descent to a great depth, however, will not necessarily result in 
any marked lithological change, as has been shown in the cases of 
the Nova Scotian and South Welsh coal-fields, where sandstones, 
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