204 A. Harher — Lamprophyres of North of England. 



closest connexion with their parent-rocks ; going to greater distances, 

 we find the total alkalies falling oif, but still far in excess of the 

 amounts proper to rocks of like silica-percentage. The relation 

 between the two alkalies noticed above is by no means universally 

 found in the lamprophyres (cf. those of Cornwall), but it is certainly 

 very common and characteristic. The Shap granite has 8-22 per 

 cent, of alkalies. In Mr. Houghton's eight analyses of the dykes 

 the figure varies from 7-99 to 4-52. The ratio of potash to soda in 

 the dykes ranges from 9-51 to 0'93, and is in every case but one 

 higher than the ratio in the granite (I'Ol). It is particularly high 

 in the most basic of the lamprophyres. 



The chemical peculiarity of the lamprophyres, as compared with 

 other rocks of like basicity, consists then in their relatively large 

 content of alkalies, and in particular of potash. The mineralogical 

 peculiarities of the rocks are, of course, simple consequences of this. 

 The abundance of potash enables nearly all the magnesia and iron- 

 oxides in the magma to be built up into brown mica, so that augite 

 and hornblende occur only as minor accessories, and original free 

 iron-ores are in very many cases not formed. A large part of the 

 potash being taken up in the mica, it follows that the predominance 

 of orthoclase or plagioclase among the ground-felspai-s will not be 

 related in any very simple manner to the proportions of the two 

 alkalies in the bulk-analysis, and a classification of the rocks based 

 on the dominant species of felspar will not be a natural one. Further, 

 in so far as any such relation holds, the plagioclase-rocks will, 

 broadly speaking, be more acid than the orthoclase-rocks derived 

 from a similar parent-magma. It may be noticed in the analyses of 

 the typical European rocks that the kersantites are more acid than 

 the miuettes. Another consequence of the abundance of the basic 

 silicate mica in the ordinary lamprophyres is that free quartz is often 

 formed in rocks with not much more than 50 per cent, of silica. 



If the more ordinary types of lamprophyres are to be regarded as 

 specialized fades of granites and syenites — the conclusion to which 

 the foregoing remarks tend — it may be asked whether other families 

 of plutouic rocks may have like modifications connected with them. 

 A few peculiar rocks have been described which might possibly be 

 considered in such a light. I would doubtfully instance Koch's ' 

 olivine-mica rock forming a small dyke in Kaltenthal in the gabbro- 

 district of Harzburg. This may be compared with the immediately 

 adjacent gabbro of Ettersberg, analysed by Streng, as being con- 

 ceivably a lamprophyre (in the extended sense) of that rock, thus : — 



Silica. Soda. Potash. 



Gabbro 50-09 1-39 0-83 



Lamprophyre?... 34-98 0-17 5-42 



Here the large quantity of potash in the second rock is very striking. 

 In the other constituents the relation of the rock in question to the 

 gabbro is that of an ordinary ultrabasic to a basic type. 



Returning to the rocks of the North of England, the question 

 naturally arises : how did the lamprophyric magma become ab- 

 1 Zeits. deuts. geol. Ges. vol. xli. p. 163 (1889). 



