of Devonshire and CornwalL 101 



strata. The rock Itself is an amygdaloid, the nodules of which are 

 chiefly calcareous, small and uniform ; the base does not effervesce 

 with acids, and when breathed upon, it has the smell peculiar to- 

 day. It is much stained by oxide of iron, which tarnishes the 

 lustre of the calcareous nodules and veins contained in it. 



Thorverton quarry is seven or eight miles north of Exeter. In 

 going to it by Cowley Bridge, on the summit of a hill which over- 

 looks Exeter, and is almost close to the town, the grauwacke is met 

 with in situ. It is of a greyish colour, with very distinct and se- 

 parate grains, breaking spontaneously into rhomboidal fragments ; 

 it is harder in some places than in others ; when tender, it splits 

 in the manner of slate, and assumes quite the character of that 

 species of rock. It occurs in strata, dipping N.W. at an angle of 

 about 70°. The same grauwacke formation is met with to the N.N.E. 

 of Exeter, on the road to Bickleigh, Sllverton, Rew, and Cross-hill ; at 

 Cross-hill the direction and inclination of the strata are very distinctly 

 seen. From Cowley Bridge the grauwacke extends as far as the 

 neighbourhood of Upton Pyne, but in this district it can only be 

 distinctly seen on the summits of the hills ; it is lost in the bottom 

 of the vallies, and is there succeeded by the red argillaceous sand- 

 stone. This is particularly the case at Upton Pyne, a village five 

 miles north of Exeter on the right bank of the Ex. As there is a 

 mine of black oxide of manganese worked in open day at this place, 

 I had an opportunity of making pretty accurate observations on the 

 strata, and on the nature of the rock. The red argillaceous sand- 

 stone, at the place where the mine is excavated, forms a stratum se- 

 veral feet in thickness from the surface ; below this is a conglome- 

 rate puddingstone, the same that is found in the parish of Heavitree, 

 but quite disintegrated : then, a reddish compact felspar in mass, 



