OF SOME ROCKS FROM GUERNSEY. 423 
difference between their felspars, and the cavities in the quartzes, as 
far as they can be compared, appear similar. 
28 (Cobo).—This rock is a little abnormal in character, but I think 
I am right in classifying it as a granite. The mineral character 
does not materially differ from that above described. The felspar is 
a good deal decomposed, but a plagioclastic species is abundant ; 
sometimes one of the latter is enclosed in a larger crystal of 
(?) orthoclase. 
20 (Dyke, Grande Maison Road Quarry).—This rock consists of 
quartz and felspar intercrystallized, exhibiting, in parts, a micro- 
graphic structure, with occasional felspar grains rather larger than 
the rest. The outlines of these are irregular, and they appear to 
be frequently surrounded by an area exhibiting the micrographic 
structure. There is also a little brown mica, an iron peroxide, and 
possibly a small quantity of tourmaline. A considerable number 
of very small cavities; most of these seem to be empty, but in others 
I detected minute moving bubbles. The rock is a vein granite. 
4 (Between Forts Doyle and Le Marchant).—This rock consists of 
quartz, felspar (orthoclase, plagioclase?, and a very little microcline), 
together with a little brown mica and iron-oxide. The rock has 
evidently been much crushed, some of its constituents, both of the 
quartz and of the mica, being of secondary formation. It is a little 
difficult to give the rock a name, as in more than one respect it is 
slightly abnormal; but perhaps it would be best to call it a granite*. 
16 (Dyke, Castle Cornet).—It consists of quartz, felspar (decom- 
posed, but apparently in great part orthoclase), and a little of a 
greenish mineral, probably replacing a black mica :—a vein-granite. 
21, 22 (Dyke, Delancy Quarry; thin seam, probably dyke, 
Grande Maison Road Quarry).—These rocks have a general simi- 
larity, but the latter is the coarser and rather more definitely crys- 
talline. The minerals are quartz, felspar, mainly orthoclase and 
microcline, and a very little brown mica and iron-peroxide. The 
rock from Grape Creek, already referred to, gives a fair idea of the 
structure, except that the felspars in the Guernsey rock are rather 
more rectilinear in outline. With some hesitation I refer it to 
vein-granite, 
46 (Castle Cornet).—Is a quartz-felsite. The ground-mass is 
erypto-crystalline, in places imperfectly spherulitic, and in it are 
scattered numerous small scales of a filmy olive-brown mineral, pro- 
bably a mica. Of the first order of consolidation are quartz, rather 
clear, showing crystalline angles, felspar, orthoclase with some plagio- 
clase, and a little brown mica. The rock, though rather more mica- 
ceous than they, reminds me of the oldest quartz-felsites in Britain. 
Hornsienpic Rocks (Drorires &c.). 
3 (Near Vale Castle).—This rock consists of a plagioclastic felspar, 
often rather decomposed, with hornblende, a rather fibrous greenish 
* [I have convinced myself that this is a form of the Lancresse granite.— 
E. H.] 
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