298 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



which catastrophe the latter rock was so shattered as to 

 form a considerable bed of granitic conglomerate, interposed 

 between the red conglomerate and the intrusive mass : here, 

 then, we have three rocks, not only produced in different 

 ways, but at three distinct periods, and yet they all gradually 

 pass into each other. 



Thus, also, according to De la Beche, " the undoubtedly 

 mechanical greywacke around Dartmoor gradually passes 

 into rocks having the characters of mica-slate next the 

 granite, as shown on a line drawn from the southern part of 

 Dartmoor to the sea, at the Bolt Head or the Prawle, a line 

 which can be examined for the greater part along the sea- 

 cliffs." 



It is very probable that the fragmentary rock has, in this 

 case, been formed of debris not far removed from the cry- 

 stalline slates ; and it may, in many parts, appear to pass into 

 the latter, because, when the parent rock is partially changed 

 by an incipient decomposition, it cannot be easily distin- 

 guished from the finer varieties of greywacke. In the cliffs 

 in Mount's Bay, in Cornwall, the felspathic rocks, for 

 instance the greenstones, become of a different colour during 

 decomposition, which of course dies away by imperceptible 

 shades, as we descend to the perfect rock : now suppose the 

 disintegrated part to be again consolidated, the transition 

 would then be perfect, and yet the upper and lower strata 

 would not be precisely the same. But this case may be put 

 in a much stronger point of view : for this bed of argil- 

 laceous debris, containing fragments of quartz and the harder 

 portions of the parent rock, is sometimes thirty feet or more 

 in thickness, and, at the bottom of the cliff, reposes on a 

 horizontal stratum of sea-sand; and this deposit tapers 

 upwards towards the hill, till it terminates in a wedge-shaped 

 mass, such as so commonly belongs to secondary strata : at its 

 upper part it is connected with the loose part of the decom- 

 posing rock just described, which has clearly not been moved, 

 for the quartz-veins continue their course therein without 



