360 Transactions.— Geology. 
It is not an improbable supposition that, where the rocks had been 
warped to a convex form by previous elevations, a compression of three 
feet may often occur in this manner in a sheet of rock 83 miles thick. We 
should have then a vein extending right across the bend, and one mile wide, 
of which the temperature was increased by 120 degrees at the surface, and 
by 10,000 degrees at a depth of 83 miles. At one-quarter of the latter 
temperature the most refractory rocks would be fused ; at the former, water 
would boil if the original temperature were 92 degrees. 
If once activity were established by an explosion of steam at the surface, 
the pressure on the heated rock below would be removed, and the eruption 
would extend to the very bottom. There is always water mixed mechanically 
with rock, and this would be raised to a temperature of 10,000 degrees. 
The tension of steam at a twentieth part of this temperature is immense, 
and if the pressure which retained it were suddenly removed an explosion 
would follow as violent as has ever been observed. 
The temperature of the lower strata is great enough to decompose the 
rocks and water subjected to it, and to account for the chemical action 
observed in eruptions. 
What occurs on a small scale at every bend in the line of fracture occurs 
on a great scale on the flanks of all lines of elevation. 
Fia.3 The ‘inelastic surface rocks are supported after 
elevation has taken place only at the line of frac- 
ture, where the elevation is the greatest, and at the 
line where the elevation ceases, as shown in Fig. 3. 
& For a moment they bear their own weight and 
then crush down tall ites can rest fully on the mass below. Earthquakes 
of great violence would be felt, and under very favouring conditions a line 
of flanking volcanoes, parallel to the main range, would burst forth. 
In all cases of surface movement, where the heat developed has been 
insufficient to cause an eruption, the rocks are heated and thus placed in a 
position more likely than before to cause an eruption when a new movement 
occurs. The rain water also percolates to them, getting heated and return- 
ing to the surface as geysers and boiling springs. 
It will appear from what has been said that volcanoes are generally not 
deep seated. The mass of liquid lava which fills a crater rests on a cold 
and solid bottom instead of being in direct communication with the fluid 
centre of the earth. In the same way if shafts were sunk into the strata 
' disturbed by the superficial action, the temperature would increase for some 
distance, but would afterwards decrease as we reached the part which had 
been affected only by the primary deep seated movement, and which had 
been increased in temperature only a few degrees, 
