136 EEY. E. HILL AND PEOF. T. G. BONiNEY ON THE 



felspar, shows characteristic cleavages, is generally of a light raw- 

 umber colour slightly tinged with green, is dichroic, changing from 

 a rather more yellow tint of the aforesaid colour to an olive brown, 

 and is darkened rather irregularly with disseminated opacite.' 

 Small crystals of apatite and grains of iron oxide (? in part ilmenite) 

 are fairly common, each, when small, being not seldom enclosed in 

 the hornblende. Both are comparatively rare in the lighter layer. 

 Sphene occurs, but is rare. The felspar in both parts is fairly idiomor- 

 phic. It exhibits multiple twinning on the albite and pericline 

 types, the former being the commoner ; that in the darker layer 

 seems less well preserved, but gives tints of a higher order, and its 

 extinction-angles seem to be larger ; those in the lighter do not 

 generally exceed 21° measured from the trace of the composition- 

 plane. Hence it seems probable that the felspars in the former are 

 labradorite, in the latter are nearer to andesine or oligoclase. In 

 the former also quartz is rare ; in the latter it is not uncommon in 

 the intervals between the felspar. Biotite is present, and its mode 

 of occurrence is remarkable. In the dark layer generally it is not 

 common, but is abundant at the junction with the light one in a 

 kind of selvage about one tenth of an inch thick, the flakes usually 

 ranging from about '01" to '03" in length, though they are sometimes 

 smaller. They cling to, and seem to penetrate into, and to be 

 formed within the hornblende grains (c/. fig. 3, p. 131), which in this 

 part as a rule are comparatively free from opacite and of a more 

 distinctly green colour. In some cases the mica actually seems to 

 replace part of a hornblende crystal.^ In the lighter layer biotite 

 also occurs, but it is not abundant ; here it is occasionally asso- 

 ciated with grains and flakes of hornblende, which have a ' residual ' 

 aspect. Both hornblende and felspar throughout exhibit a slight 

 orientation. Occasionally there are some signs of strain,^ but none 

 of crushing. A careful study of this rock has suggested the fol- 

 lowing inferences : — (1) that the biotite is not so old as the horn- 

 blende ; (2) that it was formed in part at least at the expense of the 

 hornblende, but that this took ])lace before the final consolidation of 

 the mass ; (3) that some movement, probably slight, occurred during 



^ Not a little of this consists of minute belonites ; these lie roughly at right 

 angles, and seem to have no relation to the existing cleavages. Is it possible that 

 they indicate planes which have disappeared and that the mineral originallj- was 

 augite ? As bearing on this it may be noticed that some grains of a brownish 

 mineral, one being rather large, appear in the lighter band in one slide. In 

 colour it is a light brown, but is not very clear, parts being of a slightly darker 

 tint and more dichroic than the rest ; a cleavage is perceptible, but unfortu- 

 nately does not aid much in determining the mineral. Here and there the latter 

 is associated with grains of iron oxide and a little green hornblende, and the 

 large grain is pierced by the felspar, and so of later consolidation. This 

 mineral presents some resemblance to sphene, but is more probably a varietj' of 

 augite. 



^ It may be noticed that at the selvage the felspars commonly are smaller 

 than elsewhere. 



^ In one case a crystal of felspar appears to have been ruptured by a strain, 

 and the intervening space is occupied by a felspar of somewhat different nature, 

 through which, however, the bands of multiple twinning are continuous though 

 they are less distinct. 



