200 A, Harker — Lamprophyres of North of England. 



lamprophyres of the region are exceedingly rich in brown mica, 

 ■which shows a characteristic mode of alteration by internal bleaching 

 with separation of magnetite or limonite; often also the interposition 

 of little wedges of calcite or dolomite. The mica is often accom- 

 panied by augite in well-formed crystals but usually quite decomposed, 

 and in the Sedbergh district Di*. Hatch records pseudomorphs after 

 olivine. Original magnetite may occur in variable quantity, but in 

 very many cases is entirely wanting. Apatite in fine needles is 

 universal. The ground-felspars include both monoclinic and triclinio 

 species, the relative proportions of the two not being a character of 

 importance. In the more altered rocks the felspars are not to be 

 made out at all, unless the cai'bonates have been dissolved out of 

 the mass. Original quartz occurs in the ground-mass of the lampro- 

 phyres in the central part of the area only (the Sale Fell intrusion 

 being excluded). 



Certain porphyritic elements enclosed in the general mass of the 

 rocks, despite their insignificant bulk, are of the highest interest : 

 they are quartz and felspars. Quartz is found in some abundance 

 in the intrusions very near the Shap granite ; at greater distances 

 it occurs only sparingly and sporadically, but isolated grains are 

 found even in the dykes at Cronkley in Teesdale. In the acid 

 sills and dykes near the granite the mineral forms sharply-defined 

 pyramidal crystals, in the transitional varieties of rock the crystals 

 are more or less rounded, and in the most typical lamprophyres the 

 quartz occurs in rounded blebs rarely showing any relic of crystal 

 outline. The rounding is clearly due to corrosion by the enveloping 

 magmaj and the blebs are commonly bordered by a narrow pale- 

 green rim of rather fibrous hornblende, converted in the more 

 decomposed rocks into a chloritoid substance. Isolated quartz-grains 

 with a corrosion-border of augite or hornblende are known in the 

 lamprophyres of other districts, and have usually been regarded as 

 mechanically caught up from the walls of the dyke. Such a view 

 seems to be merely an d priori one, based on the improbability of 

 original quartz-grains occurring in basic rocks, and we shall see that 

 the facts are susceptible of a different reading. It may be noted in 

 passing that similar grains of quartz with a corrosion-border of 

 augite are found in various American olivine-basalts, and are clearly 

 shown to be original constituents.^ 



Very similar in many respects are the phenomena of the porphyritic 

 felspars in our rocks. Both orthoclase and oligoclase are found, as 

 in the Shap granite. In the dykes and sills nearest the granite these 

 minerals occur plentifully ; elsewhere they are, as a rule, sparingly 

 distributed. In the intrusions in the Cross Fell inlier, for instance, 

 an ordinary hand-specimen may show perhaps one crystal, perhaps 

 none ; in Teesdale the felspars are absent, but near Ingleton, at an 

 equal distance from the granite, they occur in some of the dykes. 

 The crystals of both kinds of felspar are always well rounded by 

 corrosion in the typical lamprophyres, less markedly so in the more 

 acid varieties and the transitional rocks, and quite intact in the 

 ^ Iddings, Amer. Journ. Science, vol. xxxvii. p. 208 



