Geology of the South-east of Devonshire, 489 



possible that to some extent, both in Devon and Cornwall, granite approached the 

 surface during this comparatively early east and west disturbance. Some of the 

 blocks on the beach in Morte Bay resemble the red porphyry of Exeter, which 

 certainly belongs to the close of the carbonaceous period. 



Parallel to the range of the older strata in North Devon and the west of Somer- 

 set there occurs in South Devon a line of elevated fossihferous beds extending from 

 Tintagel to Tavistock, and forming the southern boundary of the carbonaceous 

 deposits. 



The conditions suggested by the consideration of the great slate system of the 

 west of England are those of slow and tranquil deposition, and an uniform condition 

 as to depth ; and the delicate markings which the casts of the various shells pre- 

 serve, indicate the minute state of the sedimentary matter. We can never expect 

 to ascertain the amount of dislocation and disturbance which such a system has 

 experienced, owing to its uniform mineral character ; but as we know, that in this 

 very district, all the newer formations present such phsenomena on a large scale, 

 and that such disturbances have been propagated from below, the lowest deposits 

 should exhibit the sum-total of the disturbances of every geological period. 



The included limestone bands, as at Knowle, Broadhempston, and Little Hemp- 

 ston, of which a representation is given at fig. 9, PI. XLIL, will show that with the 

 apparent uniformity, the result of cleavage, there may be much disturbance. The 

 faults and contortions of the coral hraestones may be observed in every quarry, and 

 some instances in the neighbourhood of Torquay have been represented in the 

 third volume of the Geological Transactions* by Mr. De la Beche. Near Broad- 

 sands inTorbay the red slates include a vein or bed, apparently composed of lime- 

 stone pebbles ; and though both the ground-plan and vertical section show it to be 

 broken and heaved in several places, yet at a short distance on either side, it 

 would be impossible to detect the disturbance. 



It has been suggested {see ante, p. 477) that portions of the granite of Dartmoor 

 may be of different ages : it certainly has partaken of all the disturbances of the 

 district subsequent to those of an early period, with an east and west direction ; and 

 the highest portion, such as that of the Cawsand group, seems to occur at the in- 

 tersection of two lines of elevation. The trappean bands of the slate system of South 

 Devon and Cornwall ; their abundance along the north skirts of Dartmoor, interstra- 

 tified with beds which certainly were deposited before the intrusion of any granite ; 

 the remarkable lines of erupted trap on the south, running parallel with the present 

 granitic axis of the county, and becoming less frequent as we recede from it, till in 

 the southern slate district they are nearly wanting ; and the great porphyritic mass 

 of Exeter, at the eastern extremity of the same line, seem to indicate the continuance 

 of igneous action along the same line, and through a vast period of time. 



• PI. XVIII. figs. 3, 4, and PI. XIX. figs. 1, 2. 



