190 
ME. EOEEET MALLET ON VOLCANIC ENEEGY. 
130. Assume the first 100 miles in depth of the earth’s crust to consist of 20 miles of 
the above rocks (which are all of about equal respective depth) and of 80 miles beneath 
of crystalline rocks, and of unknown acid and basic magmas, crystalline or not, of a nature 
analogous to our most rigid crystalline rocks. 
We may group the rocks of our Table I. into the newer, or Nos. 1 to 9 (viz. from the 
Oolites to the Silurian slates), and the older, from Nos. 10 to 16 (or from metamorphic 
to granites &c.). 
The mean coefficient from the first is 2449°, and from the second 5650°; and the last 
may be taken to represent all below the azoic rocks; we therefore have 20 miles in 
depth at 2449° and 80 miles in depth at 5650°, which gives a mean coefficient for the 
entire depth of 5010°. 
131. This is, however, certainly below the truth; for the Devonian limestone and North- 
Wales slates (Nos. 7, 8, and 9 have coefficients almost or quite as high as several of the 
granites and quartz rock of Holyhead, as deduced from my experiments on its crushing, 
Philosophical Transactions, 1862) have a coefficient exceeding 5316°, or equal to granite 
(see Appendix), and constitute much of the deepest 5 miles of our entire 20 miles of 
known rock; and there is to be found no doubt a large increase of metamorphic rocks 
of the Bowley Eag (No. 10) class at the lower part of the known series. 
It will therefore be a better approximation to group together all the rocks in our 
Table I. except Nos. 10, 12, and 16, which have the largest coefficients, and take the 
mean of these for the coefficient of the known or stratified rocks, and the mean of 
Nos. 10, 12, and 16 for that of all the crystalline or other rocks beneath. 
We shall then have the mean for all, except^ Nos. 10, 12, and 16, =4110°, and for 
Nos. 10, 12, and 16 = 7060°, or for the whole depth 
20 miles at 4110°, 
80 miles at 7060°, 
giving a mean for the entire depth of 6472° per cubic foot of rock. 
132. A mean specific heat obtained in the same way from those given in column 27 
gives 0T99 for the average of the Avhole 100 miles depth of shell; and from the specific 
gravities given in column 3 we deduce a mean for all the rocks experimented on of 2627. 
Taking that as the mean for 20 miles in depth, and assuming 2900 specific gravity for 
the 80 miles of denser rocks beneath, we obtain a mean specific gravity for the whole 100 
miles of crust of 2842. 
133. Whence we have the following numerical results: — 
(1) H = 6472 British units per cubic foot of crushed mean rock. 
G472 
(2) =103° Fahr. = the temperature to which 1 cubic foot of water at zero is 
raised by II. 
G472 
(3) 5 ^8 x 143 — 0’783== cubic feet of ice at 32° melted to water at 32°. 
