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429 PROF. T. G. BONNEY ON THE MICROSCOPIC STRUCTURE 
brown mica, the latter mostly in aggregated small scales. Apatite 
not conspicuous. 
8 (Vazon Bay, middle).—Felspar, rather decomposed microcline 
and plagioclase recognizable, a fair amount of aggregated chlorite 
or some similar replacement-product of brown mica, with some scales, 
rather larger than usual, of a hydrous white mica, and a few micro- 
liths of apatite. 
15 (Castle Cornet).—A considerable quantity of microcline among 
the felspar, several clusters of altered biotite, with a few scales of 
white mica and some granules of hematite and opacite. 
48 (Brickfield near Les Talbots Road).—A gneiss with fairly 
marked foliation, consisting of quartz, felspar, and a dark brown 
mica. The quartz is rather clear, and occurs generally in rather 
small clotted granules ; the felspar is in part orthoclase, but there 
is a considerable quantity of closely twinned plagioclase, extinguish- 
ing at small angles with the vibration-planes of the nicol, and pro- 
bably albite or oligoclase. The mica occurs in small plates, generally 
ageregated ; there is some opacite, and a little apatite. 
GRANITE. 
24, 26 (Around Lancresse Bay, N.W. part of island).—These 
specimens only differ in that one is rather more coarsely crystalline 
than the other. They consist of quartz, felspar, and black mica. 
The quartz contains a fair number of cavities, in many of which 
are small bubbles, often moving, but some are empty; others are 
rather dark, as if stained, and there are occasionally microlithic 
enclosures. Felspar is the most abundant mineral, usually in well- 
defined prismatic crystals, rather decomposed. Plagioclase pre- 
dominates, frequently in closely twinned crystals, and sometimes with 
external zonal banding. By measurement of the extinction-angles 
I infer that much of it is oligoclase. 1 think, however, that some 
may be albite, and occasionally recognize orthoclase. The mica, not 
very abundant, is rather dark brown, containing evidently a con- 
siderable amount of iron. It is occasionally replaced by the usual 
green mineral. There is also a very little pyrite, apatite, &c. 
(38) does not materially differ, except that it has a little more 
apatite and a crystal of sphene. (27) contains much the same 
minerals, though it is rich in quartz; but it differs in structure, 
having a ground-mass which is almost microcrystalline, in which 
larger crystals of quartz and felspar, rather irregular in outline, 
are thickly scattered; it is, in fact, an example of the granite 
porphyry of some authors. The cavities in the quartz are smaller, 
and bubbles seem to be less frequent. (5) A granite of similar type 
to those named above, but seems to be a little crushed ; fiuid-cavities 
abundant. (25) Part of the slide exhibits a similar granite, the 
rest a rock consisting of hornblende, felspar, and a little quartz. 
At first sight one would suppose it an intrusive junction; but closer 
examination leads me to think it more probably a node of some 
kind. There appears to be a transition, though very rapid, rather 
than a break between the two rocks. There is no very marked 
