282 



NOTES ON GEOLOGICAL RESULTS. 



Second point. — Aplites are usually associated with a mica 

 rock called " minette." Now the nearest " minette " I know 

 of is at St. Sampson's Harbour, and it at once occurred to me 

 that I might find that rock here also. 



Third point. — I have reason to suspect that " Eurite," a 

 different form of aplite, is to be found at Bigard on the 

 cliffs, and it would be of geological interest to have found 

 these modifications near to each other. 



Before going on with the problem, let me show the 

 differences between these rocks. 



Eurite is composed of Quartz and Potash felspar ; Aplite 

 is composed of Quartz and Soda felspar. In the same way 

 felsites are composed chiefly of Quartz and Potash felspars 

 and of Quartz and Soda felspars. The distinction between 

 these rocks is one of microcrystalline structure. 



Fourth point. — Felsites have been found all over the 

 island, hence usually they offer no difficulty, but they weather 

 brown as a rule and this rock weathered white. 



Fifth point. — In physical properties such as jointing, 

 cleavage, &c, the rock was a felsite, but there were variations 

 in structure caused by the original flux, not visible in a hand 

 specimen, which made me uncertain. 



I now give a description of the dyke. It is to be found 

 in a gully at the base of the east face of the Corbiere promon- 

 tory. The cliff outcrops everywhere with macro-crystalline 

 (phyritic) pink gneiss in which numerous greenstone intru- 

 sions occur. Side by side with the greenstone is the felsite. 

 Whether it cuts the greenstone (Hornblende-dolerite) or lies 

 conformably to it has not been determined. This composite 

 dyke is easily distinguished from the surrounding rocks by its 

 colours. It is foliated and contorted, and is weathering at a 

 greater rate than the adjacent greenstones. 



The gneiss is harder and the dykes are overhung by it so 

 that in places you pass under the overhang on hands and 

 knees. This gneiss overhang is about 60 feet from the sea 

 level. 



As the question was one of structure I forwarded a 

 specimen to the Jermyn Street Geological Museum for 

 examination and report, which report I append. 



Geological Survey and Museum, 



Jermyn Street, London, S.W., 

 25th July, 1911. 



Dear Sir, 



I beg to enclose a report on the rock from Guernsey 



