392 Mackintosh — Railway Geology in Devon. 



with miTch-rounded pebbles of qtiartz, and of a dark, bard rock with 

 quartz veins, which may be seen in situ on the borders of Dartmoor. 

 There were likewise pebbles of slate, a coarse kind of sandstone, 

 etc., but I could see no granite, though it has been foimd on Great 

 Haldon (farther north) by Sir H. de la Beche and Mr. Godwin- 

 Austen. The flints often appeared as if they had been gathered into 

 groups. In a pit on the east side of the hill the flint gravel was 

 underlaid by laminated Greensand, containing in its upper part 

 blocks of chert graduating into more or less ferruginous sandstone. 

 I failed to see beds of gTavel sufficiently distinct to indicate separate 

 periods of accumulation. 



Origin of Flint-gravel, etc. — The north and south contour of the 

 two Haldons is a horizontal line, which is generally admitted by 

 geologists to be a sure indication of a sea-bottom ; and this line is 

 roughly on a level with the table-lands of the Blackdowns. That 

 both were parts of a formerly-continuous ocean-floor, and that the 

 plains and valleys now separating them were excavated by water, is 

 likewise generally believed.^ As regards the Haldons the rounded 

 stones, some of them transported, which compose part of the gravel, 

 show that it is not a subaerial accumulation ; and Mr. Godwin- 

 Austen long ago noticed its resemblance to marine shingle. But 

 this shingle is continued down the slopes on both sides of Little 

 Haldon, showing that the sea not only formed the level summit line, 

 but likewise (during a continuance of the same submergence, or 

 during a subsequent submergence of the land) the surface of the 

 slopes. That marine denudation extended down the sides of Little 

 Haldon is also shown by the fact that (excepting where there are 

 combes) at equal depths below the summit line the surface presents 

 a succession of north and south horizontal lines to a spectator 

 situated at distances too near to admit of the summit being seen. 

 If so, where are we to fix the downward limit of marine denudation ? 

 The sides of many ridges and valleys in Devon present a series of 

 longitudinally-straight lines. Transversely they are regular geomet- 

 rical curves, which cut the more or less upturned edges of the 

 strata. This is the case, not only with the Trias between Dawlish 

 and Teignmouth, and between Teignmouth and Bishopsteignton, 

 but likewise with many limestone undulations near Torquay, and 

 between Newton -Bushell and Totnes. In most quarries and railway- 

 cuttings in south-east Devon the unweathered Trias, and especially 

 the limestone, comes up within two or three inches of the surface. 

 Some will probably regard this uniform smoothing and rounding off 

 as the work of gi'ounding and grinding icebergs. Others may lean 

 to the idea of a great flow of land ice, similar to that now moving 

 over a great part of Greenland. 



From Teignmouth to Newton-BusJiell. — The Triassic strata on the 

 right-hand side of the railway, though not much inclined, show no 

 conformity to the shape of the lateral valleys which open into the 



1 Sir H. de la Beche and Mr. Godwin- Austen both believed that the excavation of 

 the valleys commenced later than the distribution of the gravel on the ancient Black- 

 down and Haldon area. 



