SCHISTS OF THE LIZARD DISTRICT, 17 



times they are about *4 of its whole volume, sometimes much less. 

 The felspar is generally rather decomposed, hut orthoclase and a 

 closely twinned plagioclase (oligoclase or albite) can be distin- 

 guished. The mica is olive-coloured, associated with opacite, and 

 is probably an altered biotite. There is, I believe, a little tourma- 

 line, but it is not very characteristic. The dark rock consists of 

 about equal quantities of felspar (mostly replaced by secondary 

 microlithic products) and a rather platy green mineral, neither 

 mineral having externally a definite crystalline form. There is 

 also a little of a brown mica, epidote (?), ferrite, and opacite. 



Taking now a specimen (collected from a mass caught up by the 

 serpentine in the cliffs south of Polteseo, p. 6), in which the lighter 

 and darker layers may be seen interbanded in a small hand-specimen 

 (typical of much of the rock in the neighbourhood), we find the two 

 varieties opposed in the slide. The passage between them is rapid, 

 and their difference from those just described is only varietal. The 

 quartz, however, contains numerous cavities, in many of which are 

 bubbles, generally occupying a relatively small portion of the cavity. 

 They seem often to be roughly arranged in lines nearly transverse 

 to the general direction of the foliation, which here is rather more 

 conspicuous than before. In one or two places is exhibited the 

 curious micrographic structure, which has often been noted in meta- 

 morphic rocks, rudely resembling the canal-system structure of Eozoon. 

 Another specimen, " from a little below the serpentine, near the 

 arch in Polbarrow Cove, typical of much of the rock in the neigh- 

 bourhood " (p. 5), exhibits a slightly closer and more irregular 

 interbanding, as in a current-bedded rock. It contains less quartz 

 in proportion to the felspar, and hornblende in all parts is more 

 abundant than mica. The larger grains of the hornblende have 

 their axes less uniformly parallel than in the former specimens, and 

 parts of the slide come very near to some of those from the horn- 

 blendic group. A still more remarkable specimen has been examined 

 fromKennack Cove (p. 7, PI. I. fig-1). This, in a vertical thickness 

 of about 1J", exhibits four light-coloured layers separated by dark 

 bands ; two of these are only about *1'' thick, but another is nearly 

 •5", and in the upper part passes irregularly into the next layer. 

 The materials of this appear to be mostly rather rounded grains of 

 felspar, and one, about J" diameter, isolated in the neighbouring 

 darker part, heightens the resemblance of the rock to an irregularly 

 banded grit. The felspar occurs in grains very variable in size : the 

 majority, rather less than *01" in diameter, form, with quartz, horn- 

 blende, and mica, a kind of ground-mass in which grains six or seven 

 times the size, and sometimes much larger, are imbedded. The 

 amount of decomposition is variable ; some grains are excellently 

 preserved ; others, especially the larger, are quite kaolinized at the 

 centre. The boundary between the decomposed interior and the 

 undecomposed exterior band is often sharp. The former is rounded 

 or subangular in form, the exterior of the grains (both in the small 

 and in the large) being ragged and irregular, instead of straight- 

 edged as in a porphyritic igneous rock. A closely twinned plagio- 



Q. J. G. S. No. 153. c 



