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TIIE GEOLOGIST. 



valuable paper recently read before the Royal Society, showed the 

 evidence of a direct correlation between mechanical and chemical 

 force. This new phase of the direct correlation of those forces is 

 destined undoubtedly to have a wide influence on that branch of 

 geological research that attempts to explain those various phenomena 

 which are the results of rock-masses having been subjected for loug 

 periods to pressure, and during which such mechanical force must 

 have modified materially those chemical changes which were dependent 

 on the slow action of weak afiinities. 



This subject is, however, only incidental to our present topic ; but 

 just as the study of these curious " nagel-flue " pebbles has brought us 

 the knowledge of a new phase in the correlation of forces, so the 

 knowledge of the direct correlation of mechanical and chemical force 

 will soon manifest its influence in increased knowledge of meta- 

 morphic conditions. Already we have the germ before us, in Mr. 

 Sorby's observations on the presence of ripple-drift in mica-schist ; 

 for the presence of that structure shows former sedimentary origin. 



"While standing under the shelter of some rocks during a shower of 

 rain, Mr. Sorby saw, on their wetted surfaces, some markings which 

 struck him as being the more or less contorted lines of that peculiar 

 kind of deposition which, in his former accounts of the intimate 

 structure of rocks, he has called by that term. " When ripple-marks 

 are found," he says in a communication to ourselves, " whilst material 

 is being deposited, a structure is generated which I have called ' ripple- 

 drift' in various papers published on the subject. It might very well 

 happen that no fracture of this could show any ' ripple-marking,' pro- 

 perly so called, but yet it is so peculiar and has so many characteris- 

 tics, that one could not confound it with any structure produced by 

 other means. If we were to find what looked like ripple-marking in 

 mica-schist, we could not be sure that they were not some of the 

 mere mechanical bendings so common in that rock, and hence we ought 

 not to base any important conclusion on their occurrence. How- 

 ever, the structure of ripple-drift is so different from anything that 

 could be produced by any other means, that its occurrence in mica- 

 schist must be looked upon as a most convincing proof of the sedi- 

 mentary origin of that rock. In some cases the micaceous schist of 

 the Highlands of Scotland show this structure to great advantage, 

 and there can be little or no doubt of its nature ; but in other cases 

 the very great disturbances which have produced so many contortions 

 in the rock, have so modified the arrangement that it is only by care- 



