OF SOME ROCKS FROM GUERNSEY. 425 
31 (Quarry west of St. John’s Church).—A diorite, the felspar 
decomposed, the hornblende as in 6, looking in some cases like a 
secondary product, a little apatite, ilmenite, and (?) sphene. 
Hornsienpic Rocks (GABBRO-LIKE). 
The group of hornblendic rocks on the eastern side of the island, 
which are enclosed by the last-described dioritic group, agree with 
it in having hornblende as an abundant mineral, but present some 
marked differences. 
17 (Hougue a la Perre)—Has, macroscopically, a considerable 
resemblance to a gabbro; microscopically, it consists of fairly well-pre- 
served crystals of labrador felspar and hornblende, with some grains of 
iron-peroxide and a pyroxenic mineral presently to be described. 
The last two appear to have crystallized first. The felspar seems 
then to have commenced consolidation ; for some small crystals of 
it are enveloped in the hornblende, which mineral also appears to 
have adapted itself to the outlines of the larger crystals. Thus 
the larger hornblende crystals are interrupted by small crystal- 
line grains of felspar, as augite not seldom is by olivine; a few 
flakes of biotite are also included in the hornblende, which is very 
fresh-looking, strongly dichroic, with a characteristic cleavage. The 
pyroxenic mineral is difficult to identify with certainty ; it is some- 
times traversed by cracks containing a serpentinous mineral, and 
the smaller grains are almost wholly replaced by this. The cleay- 
age and general appearance, as well as the optical properties, cor- 
respond fairly well with augite; so that we may, I think, refer 
it to this mineral. The rock must be classed with the labrador 
diorites, and is evidently closely related to the gabbros. 
32 (North end of Bellegreve Bay).—Is a rock of the same family, 
but consisting only of labrador felspar (rather decomposed) and horn- 
blende, with a fair amount of iron-oxide and a little apatite. The 
hornblende contains a considerable quantity of opacite, often belo- 
nitic, and arranged along the cleavage-planes; it is sometimes 
rendered quite opaque. 
30 (Mont Crevelt).—Is a rock generally similar in character, but 
the felsparis greatly decomposed, and the hornblende looks rather 
altered. 
19 (Delancy Quarry).—Has also a general resemblance, so far as 
the two dominant minerals are concerned, but it contains a con- 
siderable amount of brown mica, and is rather rich in apatite. It 
also exhibits a slightly banded arrangement of its minerals. ‘The 
colourless mineral, interbanded with the mica, occurs exactly like 
the calcite figured by Zirkel (Micr. Petrogr. pl. v. fig. 1, cf. pl. i. 
A, iu. 3). It shows colours resembling those of a pyroxenic mineral 
with the crossed nicols, and appears to extinguish nearly or quite 
simultaneously with the mica, and with the only plane of cleay- 
age (faintly marked). 
34 (Dyke? Shore, near Mont Crevelt).—Is a much more minutely 
crystalline rock, containing micro-porphyritic felspar, in a ground- 
mass of felspar and hornblende, greatly altered. I should imagine 
