IGNEOUS ROCKS OF SOUTH-EAST CORNWALL. 21 



How imperfect are the data the map supplies may well be 

 illustrated by a reference to the Saltash area, where in all seven 

 bands of greenstone are shown, some of which clearly continue 

 across the Tamar. Now on the Devonshire side the whole of 

 the bands of this group, with the exception of the most 

 northerly, that at Ernesettle, are lavas, ashes, or tuffs, more or 

 less altered, and therefore contemporaneous with the slates 

 among which they are found. On the Cornish side lavas and 

 ashes still predominate (it is a schistose ash or tuff that forms 

 the point at the Saltash Bridge) ; but intrusive rocks occur at 

 various spots, as Cumble Tor, Grove, and Treluggan on the 

 Notter, Burraton Combe, and Wearde, close by Antony Passage. 

 At Cumble Tor there is a lava flow sepaiated from the intrusive 

 rock only by some altered slate ; and at Burraton Combe, imme- 

 diately north of Forder (Ford on the map), where one band is 

 shown to widen out, there are really two bands, the southern, at 

 Forder, being a vesicular lava. 



The rocks of Ernesettle, Notter, Burraton, and Wearde are 

 examples of the Dolerites, which form my third class. Their 

 intrusive character cannot be mistaken ; therefore they are of 

 later date than the Devonian lavas. Moreover, since in their in- 

 trusion they appear to have taken advantage of the points of 

 least resistance afforded by the plications of the Devonian series, 

 they are in all probability later than the great movements by 

 which these rocks were thrown into their present relations. It 

 remains to be seen whether a limit of youth as well as of age can 

 be assigned them. There is a chance that this may be done, if 

 we can clearly identify the rocks of this section (and of those 

 that will be treated next in order) among the constituents of the 

 few relics of our Cornish Triassic conglomerate, which I am not 

 without hope I may yet be able to do. 



The rocks to which I allude as the Gabbros have not been 

 identified in the district under that name ; and I use it chiefly 

 to indicate a relationship which I believe to exist between the 

 rocks in question and the undoubted gabbros of the Tavy area, 

 and elsewhere on the granite borders in Devon. Cocks Tor, 

 Smear Eidge, and White Tor in Peter and Mary Tavy, have long 

 been recognised as in the main gabbros, having diallage as their 

 pyroxenic constituent. Near the granite, however, this diallage 



