Ix PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



for in every view of the origin of the plutonic rocks moisture must 

 be conceived to have been present. According to the theory of a 

 cooling globe, the influence of the atmosphere and ocean must have 

 been very strongly felt in the formation of compounds by consolida- 

 tion from fusion ; while, according to the theory of volcanic heat 

 being generated from the contact of water and unoxidized bases of 

 alkalies, earths, and metals, some trace of water should appear, as, 

 in fact, it does in the minerals crystallized in lava. 



In this speculation Mr. Marshall follows out the views first pro- 

 posed by Mr. Babbage in his notices of the temple of Jupiter Serapis* 

 — views securely founded on the inevitable effect of the vertical dis- 

 placement of the interior surfaces of equal temperature by any dis- 

 placement of matter at the surface of the earth or alteration of the 

 areas of sea and land. Every day, by the removal of matter from 

 the land and the deposition of it in the sea, these isothermal surfaces 

 do undergo vertical displacement, and the masses of the earth's crust 

 are in consequence changed in bulk and changed in place. 



Materials of unlike nature are unequally affected, some expanding 

 by heat, others contracting, while thermo-electric currents generated 

 by these inequahties contribute to new resolutions and new com- 

 binations among the elements of matter. Metamorphism in a large and 

 general sense thus becomes a nonnal and a necessary phsenomenon. 



If we suppose a change of temperature of 100° Fahr. to cause ex- 

 pansion in a solid mass 500 miles across, this would occasion a change 

 of linear dimension of above a quarter of a mile in Hmestone and sand- 

 stone. If the pressure occasioned by this were relieved by one 

 vertical fault, it must be 16 miles in height; if by one general 

 curve upwards, it would have an elevation in the middle of about 8 

 miles. Though in fact neither of these assumptions as to the form of 

 the surface of relief can be adopted, they show how great is the 

 power of changing form and relative height generated by changing 

 temperature in rock-masses. It may here be mentioned that Mr. 

 A die found a rod of Carrara marble to acquire a set, and to be per- 

 manently elongated by the application of heat. Clay, on the other 

 hand, is known to be permanently shortened. ^' The expansion and 

 contraction of strata may form rents and veins, produce earthquakes, 

 determine volcanic eruptions, elevate continents, and possibly raise 

 mountain-chains f." 



The more we consider the conditions under which the mass of the 

 earth is held together, the more clear grows our conviction that 

 internal causes of change of figure, if not of volume, do really exist 

 and may be effective. Dr. Siljestrom, of Stockholm, caUs attention 

 to a cause of dilatation and contraction which must be supposed to 

 exist, or at least to have existed in the fluid mass, viz. difference of 

 temperature in the different parts of this mass. By the extinction 

 of this difference, through more complete mixture, in any given case 

 and in the same chemical substance, a change of total volume would 



* See Geol. Pi*oc. vol. ii. p. 72, Quart. Geol. Journ. vol. iii. p. 186, and Ninth 

 Bridgewater Treatise, p. 209. 



t Babbage, Ninth Bridgewater Treatise, p. 218. 



