266 



LAMPROPHYRE DYKES IN LONG SLEDDALE, 

 WESTMORLAND. 



ALFRED HARKER, M.A., F.R.S., F.G.S., 

 Cambridge. 



It is well known that numerous dykes of mica-lamprophyre 

 occur in Westmorland and on the western border of Yorkshire, 

 and they have often received attention from petrologists. 

 The examples to be noticed have not, I think, been described 

 or recorded, although they illustrate certain points of general 

 interest. 



The locality is in the upper part of Long Sleddale, S.W. 

 of Buckbarrow Crag. Here four parallel dykes cross the 

 River Sprint near a sheep-fold. The country-rock consists of 

 cleaved andesitic lavas, belonging to the Ordovician volcanic 

 series, and the dykes cut obliquely across the cleavage. Their 

 bearing is about E.N.E., which is directly towards the Shap 

 granite, four miles distant. There are good reasons for be- 

 lieving* that the whole assemblage of dykes in the district is 

 closely related to this granite intrusion, about which the dykes 

 are disposed radially. The widths of our four dykes, in order 

 from S. to N., are about loft., i| ft., 7 ft., and 8 ft., and it 

 is the first and last of these which are most worth examining. 



The most southerly d}^ke is largely of a type which is unusual 

 in this connection, a fine-grained bluish basaltic-looking rock 

 of specific gravity 2.745. A thin slice shows that it is micro- 

 porphyritic, with little crystals of felspar and augite in a fine- 

 textured ground, but the whole is too much decomposed to 

 allow of any precise diagnosis. Part of the same dyke, however, 

 is at once recognized as a lamprophyre with abundant flakes 

 of dark mica. Its specific gravity is 2.732. In a thin slice 

 it is seen to be of the kersantite type, the felspar being labra- 

 dorite. There are numerous shapes of olivine crj^stals, now 

 replaced by carbonates, but otherwise the rock is unusually 

 fresh. There is a third rock which enters into the constitution 

 of this dyke, viz., a quartz-felsite (sp. gr. 2.581). This is found 

 in the form of enclosed pieces, and its occurrence in this associ- 

 ation is significant. At various other places in the district 

 dykes of quartz-felsite and mica-lamprophyre are found 

 together in such circumstances as to suggest a close genetic 

 relationship, and there is evidence that the acid intrusion is 

 somewhat the earlier of the two. In the present case the 

 association is even closer, for it appears that the two rocks 

 were intruded successively in the same dyke-fissure. 



The most northerly dyke is of mica-lamprophyre, and 

 contains similar inclusions of quartz-porphyry. A specimen 

 of the dominant type gives the specific gravity 2.712. A series 



* Geol. Mag. 1892, pp. 199-206. 



Naturalist, 



