406 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



greenish sandstones with plant-remains. On the south side of the 

 fault are the grey and purplish argillaceous slates of the older series 

 dipping to the south-west at a high angle, while the gilts opposed to 

 them on the other side of the fault undulate off to the north ; but as 

 a whole the position of these Culm -measures is not very much out of 

 the horizontal, while the underlying rocks are highly inclined. This 

 is the most southern spot at which the Carbonaceous rocks have been 

 observed, and it shows conclusively that they once stretched consi- 

 derably further to the south, prior to their removal by denudation. 



It has been necessary to be thus minute in describing the southern 

 limit of the Culm-measures, in order that the relation between them 

 and the underhing series might be clearly brought to view ; and if 

 we trace the line on the map (PL XYI.) it will be seen that although 

 there is apparent conformabihty in some places, yet, when looked at as 

 a whole, it is obvious that the upper gTOup is not always resting on 

 beds of the same age. From this it would appear that the forces 

 which resulted in bringing up the granite of Dartmoor and the 

 Camelford Hills, and which have thrown the beds into their present 

 position, having been posterior in age to the Culm-measures, have 

 acted equally on both formations, and, in producing the major and 

 more manifest foldings and contortions, have somewhat modified and 

 obscured the unconformabihty which originally existed between 

 them. Following the lower beds as they trend up from the north 

 of Hingston Down by Stoke Climsland to the north-westward, the 

 overlying series crops out, and the lower rocks then occupy the whole 

 of the country between the granite and the Culm-measures of 

 Laneast Down, and show a clear succession from below upwards. 

 Hence the Carbonaceous rocks of Nighton and Trevage cannot be 

 resting on beds of the same age as do those of Laneast Down, unless 

 all the evidences of superposition are to be disregarded. Before 

 proceeding to the consideration of these lower rocks, however, it will 

 be well to notice briefly the relations between themselves of those 

 rocks which make up that portion of the Culm-measures included 

 between the line of outcrop indicated above on the south, Dartmoor 

 on the east, and the range of limestones which extend from Truscott 

 near Launceston, past Lifton, Lew Trenchard, and Eridestow, to- 

 wards Oakhampton, on the north. 



Fully to understand the structure of this triangular area it is 

 necessary to bear in mind that all the igneous rocks contained in it 

 are of the character of volcanic ash and lavas, and were contempo- 

 raneous with the rocks among which they are included. There are 

 compact and crystalline rocks among them, it is true ; but they are 

 associated in such a manner with other rocks of similar composition, 

 that have a vescicular or schistose structure, as to leave no room for 

 doubt upon this head ; and not a single instance was noticed of a 

 trap-rock of undoubtedly subsequent age*. In attempting to un- 



* It would be premature to attempt to define the mineral character of these 

 rocks until they have been analyzed. They appear, however, to consist of a 

 felspar which is usually green, less commonly white, and of a dark-green 

 foliated mineral which, as observed bv Boase, is something between chlorite 



