THE GEOLOGY OF MINEEAL SPEINGS. 
211 
Tuscany it was 1° F. in 18 feet. It is probable that mucli 
of this variation depends upon the varying conducting power, 
or specific beat of the different formations themselves. 
Assuming that 1° F. is the average rate of increase for every 50 
feet, it is estimated that at a depth of 20 or 30 miles every 
known substance would be in a state of fusion. Without 
entering into a discussion of this aspect of the theory of a 
central heat, ag'ainst which some very cogent arguments have 
been urged by Mr. Hopkins and others, we are clearly justified 
in accepting’ the fact that so far as our observation carries us, 
the earth’s temperature rises as we descend into it ; and it is 
no unreasonable deduction to infer that it is the source of that 
warmth which all deep-seated springs possess. The warmest 
natural springs known are the Geysers, whose temperature, as 
determined by Bunsen, is 248° F., or 36° above the boiling- 
point, at the depth of 74 feet in the tube of the Great 
Geyser. Then come the springs of Aguas Calientes de las 
Trincheras, between Porto Cabello and New Valencia, in 
South America, whose temperature is 207° F. From this 
temperature we may find springs exhibiting every gradation of 
temperature down to 33° F., or 1° above the freezing-point, 
which is the constant temperature of a spring at Walberg, in 
Lapland. The valuable source of heat which thermal springs 
contain has, in some cases, been utilized for practical purposes. 
Thus, the small town of Eaux Chaudes in Auvergne, warms 
the whole of its houses during the winter by the water of its 
hot springs, whose temperature is 176° F., and which is circu- 
lated by pipes. It is very much in keeping with the above- 
mentioned theory as to the source of the heat of thermal 
springs, that they are so frequently found in the neighbourhood 
of active or extinct volcanoes. In the former case we have a 
ready explanation of the phenomenon in the propinquity 
and constantly recurring ejection of highly-heated materials, 
by which the surrounding country must be sensibly warmed. 
In localities where active volcanic disturbance has long 
ceased, as in the Eifel district, the high temperature of 
local springs is probably due to the great depth of the chasms 
from which they arise. Sir Charles Lyell, in his admu’able 
address before the British Association at Bath, last September, 
has pointed out another important link between volcanoes 
and hot springs, in the fact that whilst the latter are 
known in many regions to be situated along lines of fault,^’ 
that is, lines along which the rocks have been rent, the former 
are also supposed to exist along similar but more extensive 
lines of dislocation on the earth’s surface. But a large 
number of thermal springs occur not so much in the primary 
rocks themselves, as in the line of junction between them and 
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