* Sw 
290 ---s:DYNAMICAL GEOLOGY. —‘([Boox III. | 


shales, clays, and coal-seams can be proved to have been once de- | 
pressed 14,000 to 17,000 feet below the sea-level, under an overlying — 
mass of rock, and yet to have sustained no serious alteration. They — 
must have been kept for a long period exposed to a temperature of — 
at least 212° Fahr. Such a temperature would have been sufficient to 
set some degree of internal change in progress had any appreciable 
quantity of water been present, whence the absence of any alteration 
may perhaps be explicable on the supposition that these rocks were 
comparatively dry (p. 298). 7 
Rise of temperature by rock-crushing.—But a further store 
of heat is provided by the internal crushing of rocks during the 
collapse and re-adjustment of the crust. The amount of heat so 
produced has been made the subject of direct experiment. Daubrée 
has shown that, by the mutual friction of its parts, firm brick-clay can 
be heated in three-quarters of an hour from a temperature of 18° to one 
of 40° C. (65° to 104° Fahr.)* The most elaborate and carefully- 
conducted series of experiments yet made in this subject are those 
conducted by Mr. Mallet. He subjected 16 varieties of stone (lime- 
stone, marble, porphyry, granite and slate) in cubes averaging rather 
less than 14 inches in height to pressures sufficient to crush them 
to fragments, and estimated the amount of pressure required, and of 
heat produced. The following examples may be selected from his 
table.” a 





‘Temperature Number of cubic | Volume of ice at 
(Fabr.) in feet of water at | 32 deg. melted to 
Rock, 1 cubic foot of 32 deg. evapo- water at 32 deg. 
rock due to work | rated into steam | by one volume of | 
of crushing, at 212 deg. rock, 
Base sgt pAgi a ___ i) a 
Osaen Stone, Oolite . ... . 8°: 004 0-0046 0:04008 
Sandstone, Ayre Hill, Yorkshire. 47-79 0°0234 0°2026 
Peat OO WAY 5 fh de 2) hie RAMS 132°°85 0:07 0°596 
Rowley Rag(basalt). . . . . 213°-93 0°109 0:925 
Granite, Aberdeen . ... . 155° °.94 0°072 0°617 
Scotch furnace clay porphyry . . 198°: 97 0:683 0°724 
Within the crust of the earth, there are abundant proofs of 
enormous stresses under which the rocks have been crushed. The 
weight of rock involved in these movements has often been that of 
masses several miles thick. We can conceive that the heat thus 
generated may have been sufficient to promote many chemical and 
mineralogical re-arrangements through the operation of water (postea, 
* Géol. Expérimentale, p. 448, et seq, This distinguished chemist and geologist has 
during the last forty years devoted much time to researches designed to illustrate ex- 
perimentally the processes of geology. His numerous important memoirs are scattered — 
through the Annales des Mines, Comptes Rendus de lV Académie, Bulletin de la Société 
Géologique de France, and other publications. But he has recently collected and re- 
published as Mtudes Synthctiques de Géologie Kxpérimentale, 8vo, 1879—a storehouse of 
information. The admirable memoirs of Delesse in the same journals should also be studied. — 
2 Phil. Trans. 1873, p. 187. 
