part 2 ! GEOLOGY OF T1IE MELDON VALLEYS. 115 



tuff-like rocks among the Lower Palaeozoic strata in several parts 

 of that island and had contributed a paper on the subject to the 

 Society in 1901. 



Dr. A. Holmes said that the subject of the communication was 



of particular interest to him, as he had spent some weeks in 

 mapping the country around Belstone, which lay immediately to 

 the east «>f the district described by the Author. He had found a 

 closely-similar upward succession of sediments dipping away from 

 the granite-contact, though, in the absence of evidence of the kind 

 demonstrated by the Author, he had regarded the 'dark igneous' 

 rocks as interbedded tuffs deposited above the calcareous series. 

 He admitted, however, that near Meldon there appeared to be 

 unequivocal evidence of their intrusive nature; but he still wished 

 to a-!; whether there was any evidence of repetition of these 

 puzzling rocks by faulting, for in the East Okement valley 

 he had found a repetition of the calcareous series and of the 

 overlying tuff, the structure and metamorphism suggesting that 

 the movement might have been due to underthrusting caused 

 by an underground extension of the Dartmoor granite. Near 

 Belstone the speaker remarked that he had found black chiastolite- 

 rocks at three different horizons : one below, and two above, the 

 calcareous series; and as all three were much below the chiastolite- 

 rocks mentioned by the Author, he asked whether similar rocks 

 had not been observed in the Meldon Valleys nearer the margin 

 of the main granite. 



Dr. K. L. SHERLOCK stated that he would have liked to hear 

 more about the stratigraphy of the region. The succession of 

 strata seemed to be similar to that in the Lydford-Brent Tor 

 district, some 5 miles away to the south-west; but a notable 

 difference was the absence of the pillow-lavas which, at Brent 

 Tor, succeeded the chert. It seemed possible that the ' dark 

 igneous " rocks might represent the pillow-lava, especially as they 

 had sometimes an agglomeratic and sometimes a tuffaceous aspect ; 

 for both of these appearances were found in the Brent Tor lavas. 

 However, the specimens on the table did not support this view. 

 The method by which a pillow-lava might come to resemble an 

 agglomerate is illustrated by the Middlesbrough slag-heaps. 

 There the molten slag is chilled during transport in the wagons, 

 and forms a hard skin to the still liquid interior. When tipped, 

 the molten slag Hows out. and sometimes may incorporate masses 

 of the rind. If a pillow-lava is poured out beneath the sea. the 

 parts in contact with the water are chilled, and the rind, being 

 burst by the pressure behind, may be incorporated in the matrix. 



Near Brenl Tor the chert was perhaps DO feet thick, and 

 covered considerable areas, apparently forming a succession of 

 outliers. Yet observed dips were usually high, and sometimes 

 extreme, although the mass as a whole appeared to have but a 

 moderate dip. This could be explained by the packing that had 

 been caused by thrusting on an extensive scale, and it applied to 

 all the local formations. The Author's account of 2000 feet of 



Q. J. G. S. No. 298. k 



