184^5.] WILLIAMS ON THE GRANITE OF LUNDY ISLAND. 69 



syenite in contact with it is softer, more schistose and finer-grained, 

 containing also more hornblende than towards the centre. The 

 soft and rotten condition of the granitic rock has no doubt obtained 

 for it the name oi pottle-stone by which it is locally known, since it 

 was formerly wrought for pipkins and other culinary vessels. 



As a granite the harder stone is durable, and capable of being 

 rendered ornamental. It may be seen in the pillars of the west 

 gateway and in the basement story of the mansion at Hestercombe. 



The slate is very variably affected by this dyke, and the syenite 

 itself is also sometimes more and sometimes less modified, but it is 

 rare to find a specimen in which the two rocks are united together. 



The granite of Lundy Island abuts directly against the slate as 

 a vertical lofty wall ; there are no granitic veins penetrating the 

 slates, and there is little alteration produced in them, except a slight 

 induration at the place of contact ; while on the other hand the 

 granite is greatly altered at the contact, and that to a distance of 

 ten or twelve feet, gradually changing from a well-defined gray 

 syenite through several varieties of hornblende rock to the condi- 

 tion of a black hornblendic trap of a somewhat schistose structure. 

 It has not however been gradually metamorphosed, or at least there 

 are appearances more perfectly syenitic on each side of the horn- 

 blende rock. 



The granite of Lundy Island occupies a dyke or chasm having a 

 north-east and south-west direction. It extends about three miles 

 in length, and has a breadth of half a mile. The vertical wall which 

 it presents ranges from the Sugar-Loaf on the east to the Rattles on 

 the south of the island. 



The author mentions another example of a true granitic dyke near 

 Drewsteignton, on the north-east of Dartmoor. This dyke is about 

 ten or twelve feet wide, and cuts the carbonaceous rocks transversely 

 for about a mile in a north and south direction. 



The author then referring to the domes and other masses of 

 granite in Devonshire and Cornwall, and the numerous small veins 

 which proceed from them, contrasts the condition both of the in- 

 jected and the bounding rock in the case of these veins with that 

 presented by the dykes just alluded to. In the former case, where 

 the veins proceed directly from the central mass, the bounding 

 rock is greatly metamorphosed, while the granite is little changed ; 

 in the latter, on the contrary (the dykes), it has been stated that 

 the igneous rock is much altered and the slates simply indurated 

 within a short distance of contact. He considers therefore that the 

 circumstances and conditions could not have been the same in these 

 two cases. 



If (as appears to have been the case) we imagine that the granites 

 of Lundy Island and of Hestercombe were fluid when the dykes 

 were formed, the author considers it difficult to explain why the 

 fluid matter did not penetrate and alter the adjacent rocks as the 

 veins of granite have done elsewhere, unless the circumstances and 

 conditions of the case were different. 



It appears to be the opinion of the author, that the heat of the 



