On the so-called Spilites of Jersey. 63 
At the base of the south face of Mont de l’Ouest, by the road to 
St. Aubyn, is another quarry.'. The rock here is a compact spilite,” 
sometimes very difficult to distinguish from a flinty argillite. As it 
differs considerably from those already described, it may be well to 
add a short account of the microscopic structure. 
This shows a fairly uniform ground-mass in which we cannot 
recognize any unaltered glass, although the chlorite and grains of 
iron oxide (probably in part ilmenite) which formed from the inter- 
stitial substance are very minute. The most distinct constituents 
are the felspar microliths which are much crowded and exhibit some 
approach to a fluidal arrangement. Occasional small patches where 
the iron oxide isin more minute and closer grains represent apparently 
a slight heterogeneity of the magma. With a high power the chlorite 
is seen to consist of very minute imperfectly rhomboidal flakes ; 
it occurs in the ground-mass, in an occasional amygdule or crack 
associated with epidote, and also replacing rare microporphyritic 
crystals. Certain of these have the shape of augite; but two 
varieties may be present, since some are imperfect or skeleton forms 
consisting of iron oxide with viridite included. Minute well-defined 
crystals of a brownish mineral are possibly pyroxenic. They exhibit 
dichroism from brown to a very pale tint, are zoned, have a marked 
longitudinal cleavage, and a straight, or nearly straight, extinction.’ 
The mineral may have been a hornblende, but as it occurs in short 
stout prisms, terminated by low-angled domes, giving transverse 
hexagonal sections, and contains brownish inclusions along parallel 
lines, hypersthene or possibly amblystegite is suggested. It is 
associated not infrequently with clustered granules of iron oxide 
or of chlorite. One crystal is long and narrow; this, however, 
might be a section along an edge; the dull polarization tints, and 
perhaps the diminished dichroism, might be the result of subsequent 
alteration. Except for the green pseudomorphs, which are neither 
large nor numerous, the rock is scarcely to be called porphyritic. 
It is locally cracked, indicating slight brecciation without material 
disturbance. 
The specific gravity of the solid purple “spilite” is 2-78, that of 
the compact “‘spilite” by the athletic club building is 2°75. These 
results and the microscopic structures show that Prof. de Lapparent 
has rightly referred the rocks to the porphyrites. Probably they 
may be ranked with the more basic rather than with the more acid 
members of the group. 
The “ Phyllades de St. Lo” were examined at several places. So 
far as was seen, they are a greenish grey argillite, similar to rocks 
found near Granville (Normandy).‘ ‘l'wo specimens obtained on the 
shore west of St. Aubin, not far from the boundary of the “granulite” 
are interesting as showing under the microscope slight contact altera- 
1 Tn it at the time of our visit, in 1888, stood the building of an athletic club. 
—T.G.B. 2 It is cut by two or three ‘‘ greenstone’’ dykes. 
3 It is impossible to obtain any definite results. 
4 Many of the rocks here are greenish to purplish argillites, sometimes rather 
flinty, which recall certain parts of the Longmynd group. 
