J. J. H. Teall — Origin of Banded Gneisses. 489 



rock which may be termed a mica-diorite, with a specific gravity of 

 about 275 ; at the other end of the series we have a rock which may 

 be termed hornblende-diorite (basic diorite, or possibly an epidiorite) 

 with a specific gravity of 2-99 ; between the two extremes we 

 find intermediate forms which may be termed mica-hornblende- 

 diorites. The specific gravity of one of these is 2-83. 



The more important minerals of the dioritic group of rocks are 

 felspar (mostly striated in the least modified rocks), dai'k mica, green 

 hornblende, sphene, iron-ores, and apatite. The felspar of the least 

 modified rocks is often somewhat turbid, and the turbid portion 

 frequently forms a kind of kernel in the centre of a comparatively 

 unaltered individual. This points to an original zonal structure. 

 Some of the diorites are porphyritic, and traces of the forms of the 

 porphyritic crystals (felspars) are preserved even in the foliated 

 rocks. 



The mica is similar to that of the granite. It occasionally contains 

 minute granular inclusions (probably zircon) which are surrounded 

 by very dark borders. The sphene is a very interesting and im- 

 portant constituent of the diorites. It occurs in perfect crystals and 

 also as grains. The grains in certain rocks form a border to the 

 opaque iron-ores. This is a feature which may be observed in some 

 of the gneissic rocks of Sutherland and the Malvern Hills. Iron- 

 ores are comparatively rare in the mica-diorites, but they become 

 abundant in the basic diorites. Apatite is very common in some of 

 the mica-hornblende-diorites. 



The felspars of the diorites show the same tendency to granulatiou 

 as those of the granite and the individual grains in the granulitic 

 aggregates are mostly untwinned. In the foliated diorites of the 

 banded gneissic series striated felspar is comparatively rare. 



Gabbro. — As this rock and the modifications which it undergoes 

 when subjected to regional metamorphism were described at the last 

 meeting of the British Association,' it will be unnecessary for me to 

 enter into details on the present occasion. SuflSce it to say that 

 every gradation from a massive rock to a gabbro-schist may be 

 observed in the district. Unaltered gabbros are, however, very rare, 

 just as unaltered granites and diorites are rare. Since m}'^ last paper 

 was read I have isolated the felspar and augite of the unaltered 

 gabbro from Coverack, and Mr. Player has kindly analyzed them 

 for me in my laboratory ; the former is a somewhat basic labradorite ; 

 the latter possesses "6 °j^ of chromic oxide, and is therefore a chrorae- 

 diopside. The hornblende of the gabbro-schist from Pen Voose has 

 also been analyzed by Mr. Player. It possesses over 10 °/^ of 

 alumina and differs from the augite also in containing more magnesia 

 than lime. It contains, however, decided traces of chromium. The 

 analyses, therefore, so far as they go, point to the conclusion that the 

 hornblende differs from the augite or diallage in composition, so that 

 the disappearance of the pyroxene and the development of horn- 

 blende cannot be ascribed to simple paramorphism. The specific 

 gravity of the chrome-diopside is about 3-22, that of the hornblende 

 1 Geol. Mag. Decade III. Yol. III. p. 48. 



