286 Correspondence — Mr. Alfred Harher — Mr. A. R. Hunt. 



is supplied by mechanical force precisely in the same way that light, 

 heat, and electricity supply energy in the endothermic changes which 

 they bring about." 



The experiments consisted chiefly in the reduction of silver, 

 mercury, platinum, and gold, from their salts. The result was 

 indicated in each case by a darkening of the powder, and in one 

 case, sodium chloraurate, the reduced gold was separated and weighed. 



In a first series of experiments the substance operated upon, 

 wrapped in platinum or silver foil, was subjected to a pressure of 

 about 70,000 atmospheres by means of a specially devised apparatus. 

 More interesting, however, is a second series of experiments, in 

 which similar results were obtained by grinding the powder by hand 

 in a stout porcelain mortar, so as to give a shearing motion. The 

 important conclusions drawn are : 



(i.) That in these experiments the mechanical energy does not 

 undergo any intermediate conversion into sensible heat. The 

 operation may be conducted slowly or intermittently, and the 

 apparatus does not become warmed. 



(ii.) That shearing stress is enormously more effective than 

 simple pressure. 



These two points are well illustrated by the behaviour of mercuric 

 chloride. A simple pressure of 70,000 atmospheres was not sufficient 

 to produce any change, but trituration in a mortar for fifteen minutes 

 caused a very evident reduction to calomel. In this instance the 

 decomposition is one which cannot he -produced by heat. 



Although the chemical transformations involved in dynamic 

 raetamorphism are of a more complex kind than those here noted, 

 it seems fair to conclude that, in so far as they are endothermic, they 

 may be brought about by mechanical force only, without the 

 intervention of heat, and that the most marked effects of this kind 

 are to be looked for where shearing stress has been brought into play. 

 St. John's College, Cambridge. Alfred Harkkr. 



Ma)/ ith, 1894. 



THE DEVONIAN VOLCANIC ROCKS OF STAET BAT. 



Sir, — In my paper " On Certain Affinities between the Devonian 

 Rocks of South Devon and the Metamorphic Schists " (Geological 

 Magazine, June, 1892), no attempt was made to define the horizon 

 of any of the Devonian rocks themselves. The schists were merely 

 referred to unaltered rocks whose position had been elsewhere 

 declared. 



In his recent address to the Geological Society, Mr. W. H. 

 Hudleston, in noticing my paper, remarks that Lower Devonian 

 diabases " in some districts are not by any means in evidence " 

 (Proc. Geol. Soc. vol. 50, p. 130). 



As a matter of fact the exact horizon of the Devonian diabases 

 in Start Bay (to which the metamorphic green rocks were referred) 

 does not affect my notes concerning them, which merely go to show 

 that the green rocks at the Start and neighbourhood are of about 

 the same age as the said Start Bay diabases, whether the latter be 

 Lower, Middle, or Upper Devonian. 



