370 



character. Sometimes they are almost granitic in texture, at other times 

 they are compact and hornstone-like in appearance. Sometimes they are 

 undoubtedly intrusive rocks, at other times they appear to pass into the 

 surrounding sediments. Their relations to the sedimentary rocks are 

 therefore similar to those of the porphyroids and may be explained in 

 a similar way. (1 ) GUMBEL states that the rocks contain orthoclase, 

 plagioclase, quartz, magnetite, brown mica and traces of altered hornblende. 



LOSSEN has shown that similar rocks occur in the district of the 

 Elbingerode synclinal in the Hartz, where they are associated with middle 

 Devonian strata/ 2 ) Five analyses of these rocks from the Hartz prove 

 that the proportion of alkali-felspar in the rock-mass varies from 7375 p.c. 

 to 92'20 p.c. and that there is also considerable variability in the relative 

 proportions of the soda- and potash-felspars ; sometimes the one pre- 

 dominates, sometimes the other. Not only do the keratophyres vary in 

 the amount and relative proportions of the two alkali-felspars but also in 

 the silica-percentage. Thus, while some contain as much as 70 p.c. of silica 

 (quartz-keratophyres) others contain as little as 50 p.c. The more basic 

 keratophyres resemble the diabases in external appearance and occasion- 

 ally become amygdaloidal. 



LOSSEN describes two varieties of keratophyre which occur to- 

 gether in a quarry in the Braune Sumpfthal ; the darker variety 

 is fine-grained and sometimes porphyritic, the other variety is dis- 

 tinctly granular and of a greenish-white or dark green colour. The 

 constituents of the rocks are felspar, biotite, augite, chlorite, iron 

 ores and apatite. The majority of the felspars of the coarse-grained 

 variety show a peculiar structure under crossed nicols which LOSSEN 

 attributes to an irregular association of soda- and potash-felspar, while 

 some of the larger felspars in the same variety may be definitely referred 

 to micro-perthite. Similar felspars occur in the darker variety. A light 

 yellow augite, allied to the augite of the kersantites, occurs in both varieties 

 but is more abundant in the finer-grained rock. The chlorite is darker 

 in colour than that of diabases and is due mainly to the alteration of the 

 augite. The iron ores, augite and apatite are more abundant in the 

 darker rock, and so also is the striated felspar. The two varieties are 

 intimately associated and doubtless belong to the same eruption, but the 

 more acid rock appears to have been the last to consolidate. 



Rocks belonging to the keratophyre-group have not yet been 

 recognized in this country, but they may be looked for in the West of 

 England. Apart from chemical analysis the peculiar character of their 

 felspars is the most useful diagnostic feature. 



The rhomben-porphyry of sou them Norway when fresh is characterized 



(1) See LEHMANN, Die Entstehung der altkrystallinischen Schiefergesteine, p. 83. 



(2) Z.D.G.G., 1882, p. 199 and p. 455; 1883, p. 21-i. See also Jahr. d. preuss. geol. 

 Landesanstalt fiir 1884. p. 30. 



(3) In the 2nd edition of his work, Rosen busch states (p. 418) that th*> rocks from Llyn 

 Padarn, which we are in the habit of calling quartz-felsitee, may be more correctly termed 

 quart/-keratophyres. 



