ON MICA TKAP DYKES IN THE CHANNEL ISLANDS 



BY REV. E. HILL, F.G.S. 



Though the main outlines of Channel Islands geology have 

 now been sketched out, much, very much remains to be done 

 before the picture can be called in any degree complete. 

 There are many interesting lines of investigation which may 

 be entered upon and followed out ; many mines which promise 

 rich returns to a persevering and intelligent worker. One of 

 these is the study of the dykes and smaller igneous intrusions, 

 especially in regard to their ages, relative and actual. In this 

 communication I only propose to invite the attention of 

 members of the Society to one small group of these rocks. 



This is the group of those which are known as the Mica 

 Traps. They are interesting from their rarity, for in England 

 they are plentiful only in one single district, that round 

 Sedbergh and Kendal ; they occur sparingly in a few other 

 regions, but in most districts are entirely unknown. But 

 they have a higher interest from the circumstance that while 

 the majority of the Channel Island rocks are of ages both 

 remote and uncertain, these are of a much later age, and one 

 which is approximately known. Their age is approximately 

 known from the researches of Dr. Barrois, who has been 

 conducting the geological survey of Brittany, which is their 

 home and centre. It can hardly be doubted that the island 

 Mica Trans are connected with the vast and numerous masses 

 on the neighbouring mainland. There they are found pene- 

 trating all formations down to the older members of the 

 Carboniferous Series, but not any newer formations. Dr. 

 Barrois accordingly concludes that they belong to the later 

 part of the Carboniferous period. 



I need not enter into their exact constitution or offer 

 technical definitions and descriptions. Suffice it to say that 

 they are usually rather fine-grained, soft, easily decomposed 

 rocks, characterised by the distribution throughout their mass 



