THE CHEESEWRINa, CORNWALL, AND ITS TEACHINGS. 145 



granite " ? Xumerous observations amongst rocks of igneous 

 origin have shown that when such planes have been developed 

 they assume a position parallel to the outer surface of the mass, 

 wliether this has been the atmosphere, or an envelope of 

 previously existing rocks into which the molten mass has been 

 intruded. In the case of Devon and Cornwall the granite has 

 undoubtedly been intruded amongst rocks of Devonian and 

 Silurian age, and may not originally have reached the surface 

 of the ground ; or only at rare intervals. This is a point on 

 which we cannot speak with certainty ; but of this there can 

 be no doubt, that such, a rock as granite, coarsely crystalline 

 in structure, could only have been consolidated from a molten 

 state with extreme slowness ; which would be the case if it was 

 overlain by large masses of previously existing stratified rocks, 

 or by outer portions of its own mass which have been removed 

 from off its surface by denudation. Molten masses which have 

 been extruded at the surface of the ground cool and consolidate 

 with comparative rapidity, and are consequently vitreous, 

 micro-crystalline, or granular in structure, although of similar 

 chemical composition to granite itself. Of such we have 

 examples in the " dolomite " of the Puy de Dome and other 

 volcanic hills in Central France. This rock has cooled and 

 consolidated at the surface of the ground ; and as a result it is 

 micro-crystalline or granular, though containing small crystals 

 of mica and other minerals. But it is owing to an original 

 envelope, or covering, of solid material, whether of stratified 

 rocks or of part of the granitic mass itself, tliat the structure 

 of the granites of Cornwall and Devon is coarsely crystalline, 

 the crystals of felspar, mica, and quartz being well developed ; 

 the quartz, however, generally occurs as the paste in which 

 the other minerals are enclosed. 



Evidences of Deyiudation. — The evidence that denudation has 

 taken place to an enormous extent over the Devono-Cornish 

 area meets us at every step along the coast, and especially the 

 north coast of this great promontory. Wherever we examine 

 the coast-cliffs along the western shore fronting the Atlantic, 

 we observe the strata rishig from the waters, generally at high 

 angles, or contorted and folded in a remarkable manner, as at 

 Boscastle, Bude Bay, and Clovelly. But whatever may be their 

 position, they invariably terminate upwards along a nearly 

 horizontal or slightly inclined plane extending inland from the 

 coast, and when viewed from a distance, looking almost like a 

 horizontal line against the horizon. This upward truncation of 

 the strata, bed over bed, means, of course, that they were once 



