672 Professor Sedgwick and R. I. Murchison, Esq., on the 



east, the siliceous beds become so much expanded as greatly to predominate over the rest, and to 

 give a character to the features of the country. Thus we cross, on the road towards Launceston, 

 great tracts of arid soil resting on gray gritstone, which lies bleaching on the surface, and reminds 

 us of the sterile ridges of a millstone grit country. 



Partly in consequence of a thickening of this bottom series, and partly in consequence of a de- 

 viation in the strike towards the south, this remarkable deposit of gritstone is carried to the south 

 of Bren Tor ; and from thence, deviating considerably towards the north, and skirting the mining- 

 field of St. Mary Tavy, passes into the high ridge of Black Down, and abuts against Dartmoor. 

 Further towards the north-east, all the bottom grits and shales of the culmiferous system come up 

 to the granite in succession, as may be seen in a traverse from Oakhampton to Tavistock ; and 

 the higher beds of the group then sweep round the northern promontory of the granite. 



In this part of their course they are greatly altered in structure, and have even been mistaken 

 for primary rocks. Their changes in structure are, however, due to two causes. 1st. They alternate 

 with many beds of trap, and have their texture greatly changed in consequence. But surely no 

 one will now contend that, on this account, they ought to be cut off from the other culm measures 

 on their strike. 2ndly. When near the granite, the whole system becomes highly metamorphic. 

 Should any one contend that the hard crystalline series between the coarse culm measures south 

 of Oakhampton and the granite of Dartmoor, belong to a distinct group ; we may reply, that the 

 hypothesis is contradicted by the strike of the beds, and, above all, by the existence of beds of 

 indurated black shale passing into Lydian stone, and black culmiferous limestone, (incontestably a 

 portion of the true lower culm group, not yet described,) which pass through the heart of this 

 altered group. And we might add, that, in the accidents of stratification, and in the existence of 

 fine seams of Wavellite in the joints of the black indurated shales, we have additional analogies 

 confirming our statement. 



The complete demonstration of the general correctness of our views is, 

 however, to be sought on the flanks of the granite hills to the south-west of 

 Oakhampton. A traverse in that direction shows us the successive members 

 of the group we are describing, as they range towards the granite one after 

 another ; and each, as it approaches the central rock, is found to undergo an 

 appropriate change. Thus the dark shales put on the form of Lydian stone, 

 and sometimes of compact felspar; and the quaitzose bands, under the same 

 conditions, are converted into a hard crystalline quartz rock. Close to the 

 granite, th'e changes are more complete; the masses become utterly metamor- 

 phic, and put on the form of white compact felspar, micaceous crystalline slate, 



castle. In their eastern prolongation, the pyritous shales near the bottom of the series become, 

 in some instances, much indurated, and in a quarry about a mile from Launceston, on the South 

 Petherwin road, are used for roofing slate. They contain vegetable impressions, and are of very 

 bad quality, being injured by disseminated pyrites. The slate of Yeolm bridge (about two miles 

 north of Launceston) is of much better quality, being derived from much harder beds, with a 

 transverse cleavage plane, producing a smooth striped slate. The rock may, perhaps, be indurated 

 by the neighbouring trap, which does not, however, appear in the quarries. It contains some 

 organic remains, and is in a much higher part of the series which we are describing than the in- 

 durated pyritous shales above noticed. 



