202 
THE GEOLOGIST. 
valuable paper recently read before the Eoyal 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 long 
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 affinities. 
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 INIr. 
Sorby's observations on tlie 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 difl'erent 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 hj care- 
