ME. ROBERT MALLET ON VOLCANIC ENERGY. 
189 
126. Cast steel in small blocks (according to Fairbairn) may stand without crushing- 
120, 000 lbs., or above 120 tons per square inch, or about four or five times the crushing- 
weight of the hardest known rocks. 
127. In column 24 of our Table the mean total work in foot-pounds expended in crushing 
each sort of rock is given, and in column 25 the foot-pounds of work needed to crush 1 
pound avoirdupois and 1 cubic foot of each sort of rock are given. 
Then in column 26 this is, by our formula reduced to units of heat for the pound 
and for the cubic foot of rock; and dividing these by the specific heats given in column 
27 and by the weights per cubic foot in column 4, we find the temperature in equal 
volume of rock; and in column 29 we obtain the number of cubic feet of water at 32° 
Fahr. that can be converted into steam of 1 atmosphere, or 212° Falir., by the heat evolved 
by the crushing of 1 cubic foot of each class of rock. 
Lastly, in column 30 we have the cubic feet of ice at 32° melted to water at 32° by the 
heat due to the crushing of 1 cubic foot of each of the sixteen typical classes of rock. 
128. These coefficients of heat and crushing-work for each class of rock may be grouped 
together in various ways, so as to obtain mean coefficients for groups of rock or forma- 
tions succeeding each other in depth. 
Let us endeavour to obtain such a mean coefficient for the entire depth of the solid 
crust of the earth of 100 miles in thickness. 
129. The following Table of the probable average depths of the known formations of 
our earth’s crust has been given by Professor S. Haughton (Geological Manual, p. 91) : — 
Neozoic 
Newer Palaeozoic 
Older Palaeozoic 
Azoic . 
Geogr. miles. 
Tertiary | 
to ' l 4-512 
Triassic 
Permian 
to 
Devonian 
4-458 
Upper j 
and Lower > 5"0S2 
Silurian J 
{ Quartz rock, 
Roofing-slates, 
Primary limestone 
i o OO 
b OOO 
Total depth = 18*385 
geographical miles, or rather more than 20 British miles. 
2 c 2 
