A. Harker — Lamprophyres of North of England. 205 



normally enriched in potash as compared with ordinary basic segre- 

 gations ? On this point some suggestions may be offered. We may 

 imagine beneath the area where the lamprophyres are now exposed, 

 or beneath the central part of it, a deep-seated reservoir of molten 

 magma which was partially separated under the action of gravity, 

 the heavier basic portion forming the lower strata. In this magma, 

 as it cooled, the earlier products of consolidation crystallized out, 

 the most important of these early products being the large crystals 

 of orthoclase. It is a known fact that felspar-crystals will sink 

 even in a basic rock-magma, and thus as the crystals formed, they 

 must have accumulated at the bottom of the reservoir, and thus 

 modified the total composition of the lower basic strata. The 

 examination of the Shap granite proves that a certain portion of the 

 quartz, the sphene, and especially the apatite crystallized out before 

 or simultaneously with the large felspars, and these too would sink 

 to the bottom. The felspars and quartz have been dissolved by the 

 basic magma, but their elements would not be redistributed through 

 the whole magma in the reservoir, except in so far as the dissolution 

 of the crystals was concurrent with their accumulation. The process 

 of solution seems to belong to a later stage, that of a relief of pres- 

 sure when the injection of the dykes took place. Since the por- 

 phyritic felspars occur plentifully in the Shap granite, we must 

 suppose that their sinking to the bottom was finally arrested by 

 a general consolidation of the magma, or at least a certain degree of 

 viscosity in any part which remained molten. Subsequently to this 

 came a partial refusion of the whole and the intrusion of the granite 

 into its present position, closely followed by the injection of the 

 acid dykes and sills and almost immediately the lamprophyres. 

 The diminutive size of the corroded felspars in the latter rocks 

 probably indicates that many others have been entirely dissolved in 

 the containing magma, and this solution is most reasonably referred 

 to the epoch of the injection of the dykes. It corresponds to the 

 " resorption " phenomena in the intratelluric hornblende and biotite 

 of many andesites, etc., effects ascribed to the relief of pressure in 

 the process of extravasation of the lavas. Felspar-crystals, as has 

 been stated, are found, though as a rule sparingly, in some lam- 

 prophyres at a considerable distance from the centre of the area ; 

 but, in view of the narrowness of many of the dykes, it seems pro- 

 bable that their transportation has been in some measure checked by 

 a sifting or filtering action. 



The above considerations may appear very speculative ; but in 

 reality, if the magma-reservoir be granted, they scarcely go bej'ond 

 known facts. If some such hypothesis be found satisfactory in the 

 area considered, it may possibly have a wider application. Certainly 

 the association of mica-lamprophyres with porphyritio granites in 

 numerous districts is rather striking. The porphyritio felspars, 

 however, must not be regarded as essential. All that is requisite 

 is that some constituent rich in alkali should crystallize out at an 

 early stage in a magma more or less separated under the action of 

 gravity. Such constituent may be in different cases a felspar, a 



