18 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 5, 



perhaps the altered slates near Lizard Head and the metamorphic 

 slates at Bolt Head might be regarded as indications of ancient strata 

 near the site of those promontories. Again, the rocks of North Devon 

 are highly mechanical in structure, and were evidently formed by the 

 degradation of some older strata. The enormous breaks and disrup- 

 tions, that took place in many parts of our Island before the deposits 

 of the Old Red Sandstone began, may easily explain the absence of any 

 large continuous deposits of Cambrian or Silurian rocks, either on the 

 north or south side of Devonshire and Cornwall. 



2. Assuming the truth of the previous statements of this paper, 

 we may believe that a great physical group (the Plymouth Group) 

 was deposited in a region which is now occupied by the most northern 

 coasts of North Devon ; and also along the region which is now oc- 

 cupied by the south-eastern coasts of Cornwall and by a part of South 

 Devon. 



3. Over this came the Dartmouth Group, ranging under the sea 

 where now are spread out the slates of this group both in North and 

 South Devon. But this Dartmouth Group was also largely deve- 

 loped on the north side of the Plymouth Group, ranging through a 

 space which is now occupied by the central and north-western parts 

 of Cornwall ; and the same group was probably spread to the north 

 of the Plymouth Group through a country afterwards broken through, 

 and partly covered up by the Dartmoor granite. 



4. Over the two preceding were deposited the two contemporaneous 

 Groups of Petherwin and Barnstaple. 



5. Within the period of the three preceding groups, there seem to 

 have been extensive tracts of elevated land, covered with ancient 

 Carboniferous Plants ; otherwise, how are we to account for the 

 great quantity of vegetable matter which, both from the north and 

 the south, was drifted into the great culm-trough of North Devon ? 

 Of this at least we are certain, that the great culmiferous series was 

 deposited in regular order on the three preceding groups. 



6. Afterwards came the period during which the great granitic axis 

 was elevated. When the elevatory movements began, and how long 

 they were continued, it would not be possible to determine ; but they 

 ended before the period of the New Bed Sandstone ; for before that 

 period, the granitic axis, and the other great mineral masses had 

 assumed their present relative position. But this granitic axis was 

 not elevated along the strike of any one of the above groups ; but, 

 commencing at the western end of Cornwall, it rose through slate- 

 rocks, which seem to belong to the older Devonian groups, and was 

 apparently continued, in association with the same groups, as far as 

 the great boss of veined granite north of St. Austell, round which, as 

 stated above, there is much confusion in the position of the stratified 

 masses. But the great granitic mass between St. Cleer and Camelford 

 rose between the Plymouth and Petherwin Groups, so that the 

 Dartmouth Group almost disappears from the section. Lastly, the 

 Dartmoor granite rose up and partially removed both the Dartmouth 

 slates and the Petherwin Group ; so that its north end abuts against, 

 and tilts up, the base of the culm-trough, mineralizing the great culm- 



