A. R. Hunt — Age of the Dartmoor Granites. 101 



my own paper). He also states that " the metamorphism (in the 

 granites) was produced after, and perhaps as a new phase in, the 

 great dynamic movements to which the contortion and cleavage of 

 the Paleeozoic rocks are due." ' And in another place we learn that 

 the fusion of the obstructive masses took place after the mechanical 

 effects produced by their obstruction had attained their maxima.^ 



So far as I can gather from a careful study of Mr. Ussher's and 

 General McMahon's papers, Mr. Ussher's almost incidental references 

 to the behaviour of the granite itself (for his main point is the 

 sedimentary rocks) have been misapprehended. 



It would almost appear, however, that Prof. Bonney's questions, 

 whilst missing their mark in Mr. Ussher, force me to take the 

 defensive, owing to my having written — " The crushing of the lower 

 rocks supplied the heat."^ This hypothesis, whether tenable or not, 

 is fairly orthodox, for, assuming that " pressure " is used here for 

 friction or crushing (the results of pressure), we may turn to Mallet 

 for experiment, and to Callaway for observation. Mr. E. Mallet, 

 P.R.S., has the following passage in the Quarterly Journal of the 

 Geological Society: — "The writer has thus shown that crushing 

 alone of rocky masses beneath our earth's crust may be sufficient 

 to produce fusion."^ Describing a particular rock, Dr. Callaway 

 writes in the same publication : — " During the metamorphisna 

 partial fusion, resulting in plasticity, took place. This effect is found 

 so often to occur where the shearing is at its maximum, while the 

 adjacent rock is merely a crushed solid, that the generation of heat 

 by the shearing-process becomes a probable inference."'' 



It may be as well to point out that 1 never suggested a crushing 

 of any portion of the Dartmoor granites that has ever been seen 

 by man. As already stated, the Dartmoor granites as exposed to 

 view are "characterized by fracture,"" and instead of indications of 

 pressure there is every sign of relief of pi'essure. Felspars and 

 quartzes are occasionally split by divisional planes and re-cemented ; 

 but I know of no instance even of dislocation, much less of crushing 

 or shearing, in the main mass of the granite. However, there is 

 nothing to pi'event the upper crust having been in a state of strain, 

 while lower horizons were under stress, so, in these latter, dynamic 

 action may well have supplemented the earth's heat in producing the 

 comparative moderate temperatures indicated by the granite. With 

 respect to the "fusion .... by a gentle stewing in sea- water" 

 referred to by Professor Bonney, it is possible my critic may have 

 misunderstood the following sentence, viz. " altered .... by 

 moderate heat in the presence of salt water."' By moderate heat 

 I merely meant a temperature not much, if at all, exceeding the 

 critical temperature of water, and the term was used relatively to 

 volcanic temperatures, which latter seem inadmissible if only because 

 they would dissociate the elements in the chloride inclusions. 



^ Proc. Somerset Archaeological Soc. vol. xxxviii. p. 217. 



2 Loc. cit. p. 208. 3 See ante, p. 99. * Q.J.G.S. vol. xxxi, p. 517. 



6 Q.J.G.S. vol. xlix, p. 422. 6 See ante, p. 98. 



' lieport Brit. Assoc. 1889, p. 670. 



