NOTES ON GEOLOGICAL RESULTS. 



283 



and wish to thank you for your detailed description of the 

 locality from which the rock was obtained. 



Yours faithfully, 



A. Strahan. 



Mons. Adolphus Collenette, 



Brooklyn, St. Martin's Road, Guernsey. 



Registered No. of Slide : E. 9165. 



Dyke, Corbiere, Guernsey. — Soda-Felsite. 

 Fine-grained pale rock with splintery fracture. In a hand spe- 

 men shows a few minute pterocrysts of felspar. 



Under the microscope it shows pterocrysts of albite set in a 

 microcrystalline ground mass of quartz with a little muscorite and 

 interstitial chlorite. The rock would ally itself with the soda-felsites 

 rather than with the aplites as suggested by M. Collenette. 



H. H. Thomas. 



25th July, 1911. 



The rock is proved to be a felsite. 



I have not yet accounted for the weathering, but I believe 

 that it may be peculiar to the soda felsite altered, under 

 the action of the sea. 



As you know " Bon Repos" Cove is just on the other side 

 of the Point. So when the Society went to the spot on the next 

 excursion I eagerly searched for an outcrop of this felsite, but 

 although the distance was only about 100 yards through 

 the cliff and I found felsite veins all over the cove, they were 

 of the ordinary colour containing potash felspar. 



Bon Repos is a cove full of geological interest and not 

 without spectacular beauty. The rock giving it the peculiar 

 sparkling black appearance is a long-grain diorite which 

 occurs in a massive dyke right across the bay. This long- 

 grain rock is dark-coloured hornblende diorite which in other 

 parts of the island seems to pass into or be associated with 

 hornblende gabbro. Tt is found at St. Sampson's, at L'An- 

 cresse, and in several parts of the Vale parish. As far as 

 I know it is not found elsewhere on the South coast. 



There are also parallel dykes of close-grained greenstone 

 (hornblende-dolerite) of fine holo-crystalline structure, offering 

 no points of difference from the greenstone of other portions 

 of the coast. 



Mr. Hill (see Quarterly Journal of the Geological 

 Society, August, 1884, page 417) thinks the long-grain here 

 found to differ from the long-grain of St. Sampson's. I can 

 find no difference perceptible in a hand specimen, and I see no 

 reason for doubting that the long-grain of " Bon Repos " 

 is identical with the same rock in the north of the island. 



