88 



ON THE GRANITE NEAR PLYMOUTH. 



stratified character is not less general in the quarries, where, although 

 there are none of those marked divisions indicative of intermissions in 

 the general depositions of the rock, the stone always comes out in 

 beds. The dip is different in different hills, but seems to have a pre- 

 vailing tendency towards east and south. 



Not only the hills are higher on the borders, but in many places 

 the granite seems harder there, and of closer texture. Hey-tor, Sheep's- 

 tor, Collard-tor, and Pen-beacon, will illustrate this, as well as Bol- 

 stack, the granite of which belongs to a more westerly projection. We 

 must except, however, as will presently appear, the hill sides in im- 

 mediate contact with certain rocks adjoining the moor. 



The hardness varies, from such as almost to defy the tool, to that 

 which falls to pieces by the blow of the hammer, or may be even cut 

 with a spade like gravel. 



The colour is not much more uniform, being generally pale grey or 

 whitish in the mass, with a shade of red or yellow ; but it is found, from 

 almost black with schorl to pure shining white ; and some occurs of 

 a rich red, superior in beauty to any Egyptian granite I have seen, 

 particularly where it contains tourmaline. 



That it is metalliferous every body knows ; tin being the most 

 common product. Copper sometimes occurs ; and, in a few instances, 

 manganese has been mentioned ; of which, as the miners have reasons 

 for concealing it, I do not know the geological relation to the rock — 

 whether it lies on the surface, or penetrates it in veins. Lead I have not 

 heard of in this formation* 



This granite is rich in schorl, and poor in mica ; consequently less 

 impregnated with magnesia, and perhaps more subject to the operation 

 of the weather, than is common to that rock : circumstances which 

 may help to explain its comparative fertility in grass. In summer it 

 feeds great numbers of cattle ; and in extremely hot and dry weather, 

 when herbage elsewhere is burnt up, assumes the appearance of great 

 verdure; its humid soil and cold atmosphere, which at other times give 

 it a pale and hungry aspect, contributing then to its fertility. 



The close crystalline texture of the rock, obliging the condensed fogs 

 and rain to run over its surface, is probably the cause that the valleys 

 are boggy ; extending in numerous cases up the hill sides and across 

 the summit. These bogs, dangerous to cattle, are not without their 

 value, being the great depositories of fuel ; an important article in 

 these bleak regions, where a fire- side is often agreeable in an evening 

 of July. The peat gradually accumulated there has attained a depth, 

 in many places exceeding twenty feet. The deeper the more it is 



