22 IGNEOTTS ROCKS OF SOUTH-EAST CORNWALL. 



is largely altered into hornblende. The same facts are observed 

 in a boss of similar rock at Lydford, and a boss at Houndall, 

 Cornwood. 



Grabbros also occur near Dartington, Hennock, Botter, 

 and elsewhere in Devon. These rocks are frequently associated 

 with hornblende schist, and this schist again passes into a hard 

 green-banded flinty rock which has been called ''ribbon jasper," 

 but which, seeing that the green portions are really qu.artz 

 charged with actinolite, may fairly be called " prase schist." 

 The reason why these points are brought out now, is to empha- 

 size the fact that precisely the phenomena here enumerated are 

 presented by the mass of eruptive rock which forms St. Cleer 

 Downs, and which if not a definite gabbro itself in mass, though 

 portions certainly are, certainly belongs to this Gabbro series. 

 To the age of this rock we have a modern limit. It is clearly 

 older than the granite because it is altered by it. In the other 

 direction we have to decide what is the age of the youngest rock 

 which it breaks through. In Devon that has been classed as 

 Carboniferous ; and if this be so — I am by no means convinced 

 on the point — there cannot, as we shall see, be much difference in 

 point of age between the gabbro and the granite itself. My 

 suggestion is that these local gabbros, being always found on the 

 granite margins, were originally deep-seated igneous rocks, 

 brought up on the flanks of the granite and exposed by denuda- 

 tion. It is of course quite possible that, as with the dolerites 

 already noted, these gabbros may not be all contemporary, but 

 the probability seems to be that they are of approximately the 

 same age. 



The Granites and Elvans belong to one series, though of two 

 periods in that series. The fact that the granite of Dartmoor 

 sends veins into unquestionably Carboniferous rocks, shews that 

 it is Post-Carboniferous. The fact that granite pebbles are 

 found in some of our Triassic conglomerates shows that it is pre- 

 Triassic. The elvans are shown to be younger than the 

 ordinary granites by cutting them ; and there are granitic veins 

 also of this later date. 



The age of the Triassic Trap of Cawsand is indicated by its 

 unquestionably Triassic characters, and by its association with a 

 small area of Triassic conglomerate, 



