
46 "  _  GEOGNOSY.  SRemate 4 
heat. Thermal springs, however, are not confined to volcanic — 
districts. They sometimes rise even in regions many hundreds of 
miles distant from any active volcanic vent. The hot springs of | 
Bath (temp. 120° Fahr.) and Buxton (temp. 82° Fahr.) in England 
are fully 900 miles from the Icelandic volcanoes on the one side and 
1100 miles from those of Italy and Sicily on the other. 7 
(3.) Borings, Wells, and Mines—The influence of the seasonal 
changes of temperature extends downward from the surface to a 
depth which varies according to latitude, to the thermal conductivity 
of the soils and rocks, and perhaps to other causes. The cold of 
winter and the heat of summer may be regarded as following each 
other in successive waves downward, until they disappear along a 
limit at which the temperature remains constant. This zone of 
invariable temperature is commonly believed to lie at a depth of some- 
where between 60 and 80 feet in temperate regions. At Yakutsk in 
Eastern Siberia (lat. 62° N.), however, the soil is permanently frozen 
to a depth of about 700 feet.’ In Java, on the other hand, a constant 
temperature is said to be met with at a depth of only 2 or 3 feet.’ 
It is a remarkable fact, now verified by observation all over the 
world, that below the limit of the influence of ordinary seasonal 
changes the temperature, so far as we yet know, is nowhere found to 
diminish downwards. It always rises; and its rate of increment 
never falls much below the average. The only exceptional eases 
occur under circumstances not difficult of explanation. On the 
one hand, the neighbourhood of hot-springs, of large masses of 
lava, or of other manifestations of volcanic activity, may raise the 
subterranean temperature much above its normal condition; and 
this augmentation may not disappear for many thousand years after 
the volcanic activity has wholly ceased, since the cooling down of a 
subterranean mass of lava would necessarily be a very slow process. 
It has even been proposed to estimate the age of subterranean 
masses of intrusive lava from their excess of temperature above the 
normal amount for their isogeotherms (lines of equal earth- 
temperature), some probable initial temperature and rate of cooling 
being assumed. On the other hand, the spread of a thick mass of 
snow and ice over any considerable area of the earth’s surface, and 
its continuance there for several thousand years, would so depress the 
isogeotherms that for many centuries afterwards there would be a fall 
of temperature for a certain distance downwards. At the present 
day, in at least the more northerly parts of the northern hemisphere, 
there are such evidences of a former more rigorous climate, as in the 
well sinking at Yakutsk just referred to. Sir William Thomson* 
' Helmersen, Brit. Assoc. Report, 1871. ? Junghuhn’s Java, ii. p. 771. 
* Professor Prestwich (Inaugural Lecture, 1875, p. 45) has suggested that to the more 
rapid refrigeration of the earth's surface during this cold period, and to the consequent 
depression of the subterraneous isothermal lines, the alleged present comparative quietude 
of the voleanic forees is to be attributed, the internal heat not having yet recovered its 
dominion in the outer crust. . 
* Brit, Assoc. Ieports, 1876, Sections, p. 3. 
