TOPOGRAPHY. xi 



partly blue and purple. The country to the west of the Tamar is thrown 

 down by the partially metalliferous faults of Calstock and Cotehele. Near 

 the latter place calcareous slate occurs.* There are two small patches of 

 Culm Measures at Penters Cross.f Near Landulph and Tamerton Foliot 

 the shores by the Tamar are principally composed of argillaceous slate, 

 wliich prevails thence nearly down to Saltash, where we find beds of 

 vesicular and schistose trap, mixed with slate of a sufficiently fine grain 

 to be worked, an apparent continuation of the roofing slates of Cann and 

 Rumple Quarries to the eastward, in the Plym Valley. About Saltash 

 and St. Stephens trappean rocks of various kinds —schistose, vesicular, 

 and compact — are found associated with the sedimentary, or occasionally 

 arenaceous, beds, X South of St. Stephens the slate and ash beds include 

 some calcareous bands, and pass under a thick series of argillaceous rocks, 

 interstratified with ash beds, which lead up to the base of the Plymouth 

 limestone. The fossiliferous slates of Saltash and St. Germans, with 

 their included volcanic ash beds, join on the Hamoaze with the argilla- 

 ceous rocks of Polbathick and Antony, and range eastward by the south 

 of Egg Buckland to Hemerdon and the vicinity of Ivybridge, where they 

 are broken through by the granite. § Variegated argillaceous slates 

 occupy nearly the whole of the country south of the Yealmpton lime- 

 stones to the coast. Near the mouth of the Erme arenaceous beds occur, 

 as elsewhere to the south of the strike of the limestone and along the 

 coast, Withm a few miles of Plymouth masses of Trap are common 

 among the slates, which seeni often to have been thrown out of their true 

 position by the intrusion. These igneous rocks vary greatly in hardness, 

 in some places being vesicular, ashy, and so soft that, after exposure to 

 to the atmosphere, pieces may be pulled away with the fingers ; whilst in 

 others they are extremely hard. In the parish of Egg Buckland there is 

 a considerable quantity of greenstone, round boulder-like masses of which 

 present themselves to the eye in various spots. At Yealmpton, imme- 

 diately to the north of the limestone, is a large mass of trap, which rock 

 also appears at Hareston, Lynham, near Cornwood, &c. 



Near Cawsand there is a small outlying patch of Triassic rocks, the 

 only one which occurs in the district, and distant fifteen miles from the 

 nearest point at which another Triassic outlier is found, on the coast at 

 Thurlestone, Associated with this small remnant of the New Red 

 Sandstone beds, which at one time occupied a wide area in the south of 

 Devon, is an interesting example of igneous rock, which has been iden- 

 tified with a felspathic trap of the Triassic series, similar to some of those 

 which occur in the neighbourhood of Exeter. 



* De La Bechk, Rep. Geo. Corn,, Devon, and W. Somerset. 



t Dr. Holl, Quart. Jour. Geol. Soc. xiv, 414. 



t De la Beche, Eep. Geo. Corn., Devon, and W. Somerset, pp. 60-63, in part. 



\ Dr. Holl, 



