J, G. GoodcMM — Augen-structure and Eruptive Rocks, etc. 21 



crystalline matters under notice, our chief difficulty in attempting to 

 explain these facts lies in accounting for the generation of the 

 conditions necessary for fusion, at isolated centres within a rock, 

 while the matrix, consisting of the very same mineral matter, should 

 remain unfused. The molecules, in some way or another, have been 

 locally placed under such conditions as to permit of their taking up 

 different positions from what they had when first acted upon ; and 

 yet the conditions referred to have been in action only at certain 

 points, or along certain zones, contiguous to others that have not 

 been so affected. It seems obvious from this consideration that the 

 result cannot be due to the simple action of heat from an external 

 source, such as would arise either from contact with molten masses, 

 or from any heat proceeding from the earth's interior. 



This and other reasons of the same kind have led the author to 

 advocate the view, previously advanced by others, that the chief 

 cause of the high temperature prevailing within the earth's crust 

 lies in the conversion of the motion arising from earth-creep into 

 heat. It is assumed that the heat generated in this way augments 

 with depth below the surface, until a temperature is ultimately reached 

 which is higher than that necessary to fuse the most refractory of 

 the rock constituents. Amongst many other causes that contribute 

 to bring about such differential movements of the earth's crust, and 

 therefore to generate subterranean heat, the author is disposed to 

 rank lunar attraction as one of the chief. Where earth-creep takes 

 place under comparatively small superincumbent pressure in the 

 presence of water, the rocks are simply deformed, crushed, or 

 ground, or otherwise affected, by causes which are chiefly of a 

 mechanical nature. At greater depths differential movements give 

 rise to what Teall has aptly termed plastic deformation— the rock 

 material being actually made to flow in the direction of least 

 resistance, so that what may be termed dynamo-fluxion structure is 

 impressed upon the rock. 



Good examples of this plasticity of solids when compelled to 

 change their form under great pressure are afforded in the mechanical 

 arts in the process of moulding from the solid metal the nickel 

 bullet cases in use in the British army. A " burr " of sheet nickel is 

 forced, cold, through a succession of annular dies, the first one giving 

 rise to a shallow cup with thick sides, while the passage through 

 each successive die increases the depth of the cup and lessens the 

 thickness of its sides, until in the end a tube of solid metal of the 

 required thinness and length is fashioned. The rate of movement 

 and of pressure have to be carefully adjusted to each other so as to 

 obtain the requisite degree of plasticity without permitting the 

 motion to be converted into a degree of heat sufficient to produce 

 actual fusion from the pressure employed. 



In the case of subterranean movements affecting rocks at great 

 depths, the temperature, generated by the conversion of the motion 

 into heat ,must generally be more than sufficient to bring about the 

 fusion of even the most refractory of the rock-forming niinerals. 

 When subjected to the great undulatory movements which have 



