OF SOME ROCKS FROM GUERNSEY. 421 
Bubbles are not seldom present, but a good many cavities appear to be 
empty. The felspar is usually somewhat decomposed and occa- 
sionally so much so as to make it difficult to conjecture the species. 
Four specimens contain rather abundantly very characteristic micro- 
cline; there is always a considerable quantity of a closely twinned 
plagioclase, which generally, so far as one can infer from its extinc- 
tion-angles, appears to be oligoclase ; but it is very probable that, 
albite is also present. Orthoclase is no doubt present, but the 
characteristic Carlsbad twinning does not appear to be frequent. 
1 (Fermain Bay).—Contains a considerable quantity of microcline. 
The third mineral in this specimen is a brown mica, generally in 
ageregates of rather small crystals, though now and then larger 
occur, one being about 0°12 inch long. Occasionally a little of this 
is replaced by the pale green mineral which frequently appears as 
an alteration-product of the magnesian micas, but most of it is in 
excellent preservation. The slide contains a considerable number 
of crystals of apatite, generally associated with the mica. Near to 
the largest mica crystal is a group of three (besides some smaller 
ones) cut almost transversely to the principal axis, the largest of 
which is rather more than 0:01 inch in diameter. The rock appears 
to have been a little crushed and recemented. 
.2 (Bec du Nez).—Consists chiefly of quartz and felspar (rather 
decomposed), with which are small films and clusters of a green 
mineral, associated with a little opacite and ferrite, and a few scales 
of a white mica, giving bright colours with polarized light. This is 
probably a hydrous soda or potash mica, but the green mineral is 
probably a hydrous magnesian mineral replacing biotite. 
42 (Pert Bot Bay).—The felspar is rather decomposed, but a 
closely twinned plagioclase evidently predominates. There are 
ageregated patches of a scaly green mineral of the chlorite group, 
strongly dichroic and probably uniaxial, associated with apatite, 
opacite, and very little white mica, as in the last. 
13 (Petit Bot; pink band, possibly a dyke (H. H.)).—Felspar 
rather decomposed, but plagioclase as above, with a little microcline 
and orthoclase recognizable. Some grains of iron peroxide, hematite, 
and perhaps a little magnetite ; except one or two very minute scales 
of a brown mica, no other mineral present. The figure in Fouqué 
and Levy (plate vii. 2), granulite from Grape Creek, Colorado, will 
give an excellent idea of this rock. Many would unhesitatingly 
claim this rock as igneous, in which case it would best be called a 
pegmatite, but I feel doubtful on the point. It may be a vein 
granite, but I should not be surprised if field evidence showed it to 
be a metamorphic rock (gneiss granulitique). The difficulty has 
been enhanced by the rock having been crushed in situ. 
11 (Lerée).—Rather akin to 42, but the replacement product of the 
mica is less abundant and characteristic. One crystal, about 0:07 
inch long, resembling a mica in form, is almost wholly taken up by 
opacite. The rock appears to have been crushed. 
9 (Vazon Bay, Southend).— Rather poor in quartz, rich in plagio- 
clase. A fair amount ef very characteristic hornblende, as well as 
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