THEORIES REGARDIXG THE EARTH's INTERIOR. 237 



In one place, therefore, the rate of increase may vary from 

 1° F. for every 24 feet to 1° F. for every 116 feet. 



In the deeper-seated portions of the earth's crust, where the 

 pressure is so enormous that sohds become potential liquids 

 and tiie molecules of which they consist have the mobility of 

 those of liquids, silicic acid or silica becomes active. The heat 

 of combination is equal to that of nitric acid, and wherever 

 this silicic acid comes in contact with a carbonate it replaces the 

 carbonic acid, forming tirst the various lime silicates, wollas- 

 tonite, garnet, epidote and so forth, but later, if the process 

 is continued, sending the bases out in solution and entirely re- 

 placing the original compound. The heat-equivalent of all 

 these reactions can be worked out, and in the near future it 

 will be possible, if the history of a piece of metamorphic rock 

 is known, to say that it has given out or absorbed so many 

 calories. 



The important fact to observe is that chemical reactions may 

 add to or subtract from the heat of the earth's crust. Gener- 

 ally it may be stated that at great depths the reactions cause 

 diminution of temperature, whereas nearer the surface they 

 are of such a nature that they cause rise of temperature. Every 

 reaction has its own heat equivalent, but it may be safely 

 stated that the majority of heat producing reactions stop at 

 five miles depth in the earth's crust, and that the nearer the 

 surface the greater the number of such reactions. From 

 which It follows that rocks once deeply buried, but now un- 

 covered, are rocks such as have been formed under conditions 

 of heat absorption, and we arrive at the apparent paradox 

 that the fundamental granite and gneiss was melted under 

 conditions of great cold. It is impossible in a paper of this 

 nature to go over all the reasons which make such a fact ex- 

 tremely probable ; suffice it here to call to mind that the liquid 

 condition of a substance means that the mass offers no resist- 

 ance to distortion ; hence under enormous pressures it is cjuite 

 possible to liquify a solid, stich as granite, in much the same 

 way as one can cause lead to flow under pressure*, and in 

 that case, the molecules having the mobility of that of a liquid, 

 v.iii cause the granite to have all the features of a rock crystal- 

 lised from igneous fusion. From field evidence of the contact 

 of granite and slate, for instance, it is abundantly clear that 

 the rock cannot have been at the temperature necessarv to 

 cause it to melt under atmospheric pressure ; in the classical 

 examples at Huelgoat in the department of Finisterre, in 

 Brittany, the Silurian slates on the west of the granite are 

 entirely unaltered, though on the east side they have suffered 

 extreme metamorphism, proving that it is not the heat of 

 the granite that has to do with the metamorphism, but rather 

 the pressure which happened to be concentrated on only one 

 side of the granite. 



* The parallel of granite and lead under pressure is not strictly a true one 

 since the granite becomes molten through the solvent action of water, which, 

 under high pressures, dissolves silicates. 



