Bernard Hobson — Permian Breccias of 8. Devon Coast. 317 



By polarized light it is not isotropic, but cryptoorystalline, 

 and in some parts miorocrystalline, in which case it consists of 

 quartz and felspar. I have written to Mr. E. Hansford Worth 

 to inquire whether he knows of any definite locality where this 

 rock occurs in situ. He has kindly sent me a specimen of the 

 felsite from the Slope of Legis Tor (N.N.E. of Plymouth), long. 

 4° 0' 23", lat. 50° 23' 20|", described by him.' It bears some 

 resemblance to the red quartz-porphyry of the Permian breccia, but 

 it differs in containing " frequent spheroids and aggregates of 

 minutely crystalline schorl," in its ground-mass being more granular 

 (distinctly crystalline) and salmon-coloured instead of compact and 

 chocolate red, and in having, according to Mr. Worth, a specific 

 gravity of 2-57, whereas I found for a specimen from the (sectioned) 

 block in situ in the conglomerate at the Ness 249. Mr. E. H. Worth 

 writes to me : " But from Legis Tor there comes another variant. The 

 same red ground-mass. Porphyritic quartz well developed, and cream- 

 coloured felspars numerous and prominent. Kaolinize these felspars 

 and you have one of the Teignmouth rocks at once." Though I doubt 

 whether an exact match would be thus obtained, I think it likely that 

 when Dartmoor is more thoroughly known such will be found. The 

 nearest part of the granite mass of Dartmoor is nine miles from tlie 

 Ness. In 1892 I mentioned'^ that "A mass of genuine quartz-porphyry 

 measuring about 4 feet by 1 foot is exposed on the west side of the 

 road opposite to the entrance gate of Dunchideock House [3| miles 

 south-west of Exeter], but it appears to belong to the New Eed 

 Breccia." I have no note as to whether this was the typical red 

 quartz-porphyry, but it is only 4-^ miles from the Dartmoor granite. 



III. — The Possibility op Teansport by Watbk of the largest 

 Blocks observed. 



If we assume, for the moment, that the blocks in the breccia and 

 conglomerate have been transported by water, it is of interest to 

 calculate the velocity of the current required to move the largest 

 block of quartz - porphyry observed near Bundle Head. The 

 dimensions of 5 ft. 4 in. by 5 ft. by 3 ft. 6 in. give 93^ cubic feet. 

 Taking its specific gravity at 2-49 (that of the red quartz-porphyry 

 block in situ in the conglomerate) and the weight of a cubic foot of 

 water at 62-4 lbs., we obtain 6 tons 9 cwt. 1 qr. 26 lbs., or, if we 

 deduct 10 per cent, on account of irregular form, 5 tons 16 cwt. 2 qrs. 

 4 lbs. for its probable weight. Prof. A. Penck '• gives a formula 

 from which the velocity of current at which a cube of rook with 

 a given length of side I will just remain motionless on a horizontal 

 surface can be calculated ; it isi7 == 3-86 Vl. If we take the contents 

 of the block to be 93 cubic feet it would correspond to a cube of 

 4-531 ft. or 1-381 metre side. The required velocity is 22-33 kilometres, 



' "The Petrography of Dartmoor and its borders" : Trans. Devon. Assoc. Adv. 

 Sci., vol. xxxiv (1902), p. 519. 

 * Quart. Journ. Gcol. Soc, 1892, p. 506. 

 3 " Morphologic der Erdoberfllacho " (1894), vol. i, p. 280, line 18. 



