SURROUNDING THE LAND S-END MASS OF GRANITE. 421 



by a vein partly filled with this green substance, chlorite, and a little 

 imperfectly crystallized blue tourmaline. 



This specimen was originally rather vesicular, as it contains many 

 well-defined cavities filled with the green substance. 



At a short distance from the place just mentioned there is a new 

 quarry in the same mass ; but the rock is in a different stage of 

 alteration : the flocculent green substance is here replaced by the 

 usual dull green hornblende ; and the slice is traversed by a quartz- 

 vein containing numerous blades of actinolite, with several blue and 

 brown crystals of tourmaline. 



Altered Dolerites from other localities. 



It will now be interesting to compare these Penzance rocks with 

 others having similar penological relations ; and a reference to the 

 map will show that such may be found on the east and west sides of 

 the Dartmoor granite. 



Tavistock. — At Peter Tavy the rock forming Smear Eidge and its 

 prolongation to Cock's Tor, near the granite, is an altered gabbro or 

 dolerite, some specimens having a coarsely crystalline texture, while 

 others are fine-grained ; both varieties are rather less altered than 

 the Penzance rocks, and are therefore important for comparison. 



In the coarsely crystalline variety the alteration is of precisely 

 the same character as that described in the Tolcarn rock : the dial- 

 lage has been partially converted into hornblende, and this substance 

 also fills veins and former cavities ; the felspar, though highly 

 altered, may be readily recognized ; and there are the same pseudo- 

 morphs after magnetite. 



In a fine-grained specimen of the same mass the pyroxene (appa- 

 rently augite) is very well preserved, but the felspar is completely 

 decomposed, being represented by pale green pseudomorphs ; the 

 same substance also fills cavities, several of which contain groups of 

 radiating blades of tremolite. 



Brentor, four miles north of Tavistock, presents many of the 

 features of a volcanic mass, being chiefly composed of purple bedded 

 ash, together with scoriaceous and compact trap of a greenish-grey 

 colour. 



The latter is an altered basalt, in which the augite occurs in small 

 well-formed crystals ; it has suffered little or no alteration, while 

 the felspar is converted into pale green pseudomorphs. 



The mass of " Greenstone," half a mile north-east of Brentor, is 

 chiefly ash of the same character as that of Brentor itself. On the 

 east side of the Dartmoor granite there are also several masses of 

 the so-called greenstone, of which the following may suffice as an 

 example. 



Hennock, N.E. of Bovey Tracey. — This mass and the two adjacent 

 bands of trap are altered dolerites. Near Hennock, at the end 

 furthest from the granite, some specimens of the rock have a well- 

 marked porphyritic texture, many rather large crystals of dull white 

 felspar being scattered through the mass. Under the microscope 



