300 



THE GEOLOGIST. 



strata^ as the case might be; under certain circumstances, 

 he contended that these strata, and the inner concentric 

 zones of mica-schist, and the gneiss would be invaded by 

 ramifying and anastomising veins emanating from the in- 

 ternal fluid, to an extent proportionate to the temperature ; 

 these veins would convert the zone of gneiss into incan- 

 descent lava, the mica-schist into gneiss, and a proportionate 

 thickness of the sedimentary strata into mica-schist 5 and if 

 the vis a tergo of heat should be maintained, such transfor- 

 mations would progressively advance till the superincumbent 

 or outermost strata being reduced to their point of least 

 resistance, they would necessarily yield to the pressure or 

 expansive force of the augmenting volume of the hquid 

 matter, and present all the phenomena of a crater of 

 elevation. From the whole amount of his observations, 

 taken round the granite of Dartmoor, Bodmin Moor, &c. 

 he considered that if Von Buch had not proposed the 

 theory of " Elevation Craters,'^ geologists would eventually 

 have been constrained to have recourse to some hypothesis 

 of the kind to explain the appearances presented by those 

 granitic domes. A series of specimens might be gathered 

 from many localities in South Devon and Cornwall, which 

 would show an insensible transition from the coarser volcanic 

 grits and breccias into the finest clay slate, every variety of 

 which he had traced up to those more typical products. 

 Mr. Williams stated, that his inquiries had resulted in the 

 conviction that granite gneiss, mica-schist, clay, slate, &c., 

 are no evidence of age, or position, in the geological scale, 

 but that they appertain to all formations, from the most 

 ancient to the most recent; he considered gneiss and mica- 

 schist were not simply " metamorphic" rocks, but rocks in 

 a particular or definite stage of fusion ; and, he therefore, 

 suggested that they should be termed intermediate products, 

 and granite, porphyry, trap, breccia, grit, ash, chlorite, talc, 

 and clay slate, immediate products of volcanic action. 



