SCHISTS OP THE LIZARD DISTRICT. 15 



felspathic material. Examined microscopically, the hornblende 

 generally is seen to be the most abundant material; the size 

 of the crystalline grains varies from microliths up to (occasionally) 

 •04'' diameter ; the former, however, are generally isolated among 

 quartz, felspar, &c. Where the hornblende occurs in patches the 

 grains are of a moderate size. These are almost always rather 

 irregular in outline, only the microliths exhibiting externally a 

 crystalline form, but the characteristic cleavage parallel to go P is 

 remarkably well developed. The usual colour is a sap-green ; the 

 mineral is strongly dichroic, changing from a deep tint of this to 

 a straw-yellow. In some of the larger grains enclosures are common : 

 these are of two kinds — one a filmy transparent mineral, apparently 

 of the same colour with the surrounding material ; the other a dark 

 brown, almost opaque mineral, which sometimes assumes a prismatic 

 form, at others is a mere dot ; perhaps this is only an iron oxide ; 

 occasionally the crystal is full of these. Iron-glance of larger size, 

 pyrite and magnetite are present ; the felspar grains are usually 

 irregular in outline, and so far kaolinized that it is difficult to con- 

 jecture the species, sometimes, however, the striping of plagioclase 

 is still visible. The epidote is granular in form, of a pale yellow 

 colour, and generally gives brilliant tints with crossing Nicols. Well- 

 characterized apatite is rare; sphene, or a mineral much resembling 

 it, rather frequent. 



I proceed to notice the microscopic structure of some of the 

 varieties of this rock. A specimen of the normal dark schist, not 

 markedly foliated, was described in my last paper; of those exhibiting 

 more definite structures, we may take the following : — (1) Rock from 

 Hot Point, with some indications of current-bedding and angular 

 spots of whitish felspar simulating fragments (p. 4), chiefly horn- 

 blende and felspar, with some granules of an iron oxide and a little 

 quartz, columnar microliths pale or colourless, some doubtless horn- 

 blende.. Small grains and imperfectly formed crystals, sometimes 

 aggregated, sometimes included in a hornblende crystal, of a very 

 pale brown " granular-gummy "-looking mineral (if the term be 

 permitted). Some almost certainly sphene ; but of some all that I can 

 ascertain is that the mineral is monoclinic or triclinic, and ex- 

 tinguishes at an angle of about 15° with its longer diameter. The 

 larger grains of felspar sometimes have their outline rather sharp, at 

 others rather interlacing with the matrix. They are generally much 

 kaolinized,but here and there the twinning of plagioclase is preserved, 

 the angle between the direction of extinction of adjacent layers not 

 exceeding 15°, so that it is probably albite. The same is exhibited 

 by one or two of the better-preserved grains in the matrix. An 

 indistinct irregular banding is visible in the slide. (2) Specimens 

 of the dark schist with white " eyes " from Porthoustock (p. 8). 

 Traces of foliation ; minerals rather more aggregated than in the last ; 

 a little more quartz. Some microliths like those just described, but 

 less numerous and paler in colour. A granular structure, as in a 

 quartzite, rather marked in the transparent patches. Among the 

 hornblende are occasional grains, nearly *05" diameter, having rather 



