172 Observations on some Metamorphic Rocks 



to make immense curves, doubling back upon itself and making 

 large folds, taking most wonderful forms. In fact, I can com- 

 pare it to nothing else than the contortions of strata in the 

 largest of the Cyclopian Islands as described by Sir Charles 

 Lyell. The schist always contains the little crystals before 

 alluded to, is very brittle, of a dull green colour, and always 

 preserving that beautiful silky appearance. Now, as such con- 

 tortions are known to have been caused by heat and volcanic 

 action in the Cyclopian Islands, we may reasonably argue from 

 analogy, that the same cause has operated here, though not 

 apparent. In no part of the slate, wherever it is met with, is 

 it entirely free from alteration, for in the valleys it is sometimes 

 met with, where the strata become crystalline (eurite), which pass 

 into slate and again into eurite alternately, for some 

 distance. The crystalline portions bear the marks of stratifi- 

 cation, with segregated veins of quartz, and the slaty parts are 

 intersected with veins of carbonate of lime, running at right 

 angles to the plane of stratification. Here then, we have some- 

 thing which throws light upon the subject of inquiry. We 

 have evidence first of a force which upheaved the slate into its 

 present position. A force distinct from that which raised the 

 hills, because it has been exerted in a more northerly direction. 

 Secondly, we have evidence of heat which altered the strata so 

 raised. We are sure that the heat was subsequent to the up- 

 heaval, because it caused the veins of segregation partly, if not 

 entirely, which run through several strata in an almost unbroken 

 line, and if upheaval occurred afterwards, such line must neces- 

 sarily have been disturbed, which is not the case. There can 

 be but little doubt, that the heat which altered the slate, also 

 crystallised the bands of stone, but whether this was before the 

 upheaval of the hills, on which the most of them run, or after- 

 wards, we have yet to inquire. 



The theory which I am inclined to adopt for the origin of 

 the "bands," would be in favour of the heat having preceded, 

 and the best explanation of the reasons on which this is 

 founded, will be to give the theory. Supposing, after the up- 

 heaval of the slate, and before it had commenced to decompose, 

 certain portions of it running in parallel lines were exposed to 

 the influence of caloric coming from the depths of the earth, 

 either by longitudinal and parallel cracks in the crust over- 

 laying some subterranean igneous mass, or by the flow of 

 currents of trap in underground channels. The rock so exposed 

 would be crystallised, and the surrounding strata more or less 

 affected, provided we can suppose a narrow opening happening 



