A. Sarker — Lamprophyres of North of England. 201 



normal quartz-porphyries. Within a mile of the Shap granite the 

 sills and dykes sometimes enclose large flesh-coloured crystals of 

 orthoclase identical with those in the granite itself, but more or 

 less rounded as in the dark basic patches of the granite. There is, 

 however, a significant difference. The large orthoclase crystals in 

 the dark patches of the granite have a corrosion-border of plagioclase 

 and quartz : in the lamprophyres this feature is not found, but on 

 the other hand the rounded crystals of oligoclase are often bordered 

 with orthoclase. The enveloping magma was in the former case 

 rich in soda, in the latter case rich in potash. It is likely that in 

 other districts special mineralogical relationships exist between 

 lamprophyres and the plutonic masses near which they occur, but 

 unfortunately the rocks have rarely been studied from this point of 

 view. Doss,^ in describing the lamprophyres of Dresden, remarks 

 that they enclose orthoclase crystals similar to those of the well- 

 known Plauen'schen Grunde syenite, but rounded by corrosion, and 

 these he regards as mechanically caught up from the syenite. Since 

 the dykes which he studied actually traverse that rock, the expla- 

 nation is of course a possible one, but it does not appear from his 

 description that the crystals have the form of broken fragments, and 

 the case may well be a parallel to that of the Westmoreland rocks. 



Having in common the general features outlined above, the north- 

 country lamprophyres still show very considerable variations. The 

 silica-percentage in Mr. Houghton's eight analyses ranges from 60 

 to less than 40, that of the Shap granite being 69. The figures for 

 the more basic rocks are necessarily unsatisfactory, owing to extreme 

 decomposition, some examples having nearly 30 per cent, of car- 

 bonates. The associated acid intrusives and transitional varieties 

 occur well characterized in the centre of the area and to a consider- 

 able distance from the granite, but they do not extend so far as the 

 true lamprophyres. They are well developed in the Cross Fell 

 inlier ; but some of the acid rocks there are not demonstrably con- 

 nected with the post-Silurian intrusions, and are possibly Ordovician. 

 In the Sedbergh district the lamprophyres and the acid intrusives 

 are quite distinct, though closely associated, and Mr. Strahan remarks 

 that the former intersect the latter. Eosenbusch makes the same 

 observation in Alsace, and it is probably of some generality. The 

 two sets of rocks, though genetically connected, were derived from 

 different portions of the heterogeneous parent-magma, and the 

 general rule appears to be that the injection of the quartz-porphyries, 

 microgranites, etc., slightly antedated that of the lamprophyres. 

 Where transitional varieties occur, we may suppose either that they 

 were supplied from an intermediate portion of the magma-reservoir, 

 or that an intermixture of the acid and lamprophyric magmas took 

 place during the injection. The striking variability of the rocks in 

 some localities must be due to the latter cause, for in some cases the 

 commingling of the two magmas has been very incomplete. A 

 dyke near Gill Farm exhibits abrupt transitions from quartz-por- 

 phyry to laraprophyre, such as admit of no other explanation than 

 i Tsch. Min. Mitth..(2) xi. p. 27 (1890). 



