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418 REY. E. HILL ON THE ROCKS OF GUERNSEY. 
as dykes; but an analogous rock on the north shore of Bellegreve 
Bay seemed to pass into the “ Birdseye.” 
I have noticed no olivine and no unalterered augite in any rock 
of Guernsey. 
Mica-Trap.—Two examples of this group have been discovered. 
One is a narrow dyke cutting the rocks on the shore at high-water 
mark, some way south of Bec du Nez, not far from the Doyle Column. 
Some strings of earthy matter in the cliffs near may belong to this, 
but are too decomposed to be certain about. The other, six or eight 
feet broad, crops out on the shore at Moulin Huet. Both are de- 
scribed by Prof. Bonney in the appendix (p. 426). As this group of 
rocks is rather rare, I may add that a dyke belonging to it runs up 
the cliff at the north-east corner of Port du Moulin in Sark ; a very 
narrow and rotten one cuts a cliff on the east of Jethou; and large 
micaceous blocks, evidently fallen from the cliff, lie under the arch of 
the Creux de Vis in Jersey. These are all I know of. The rock 
decomposes deeply, so that it is easily overlooked. 
Veins.—I would restrict this name to fissures filled with matter 
by segregration, sublimation, decomposition, or infiltration. I 
would withdraw it from all molten intrusions even of granite. 
Guernsey affords a fine field for study of the distinctions between 
these two classes. Can any infallible criterion of difference be -laid 
down? The quartz in a vein is often milky, and almost always 
has some peculiarity of aspect; it is, I fancy, the best indication. 
A radiation of crystals from the bounding walls, a medial line 
of union between opposite growths to a common centre, are 
conclusive when they occur, which is seldom. Great and irregular 
variation in the size of component crystals is also more likely to 
happen in these cases than in the cooling of a molten magma. 
This last feature is well seen in a vein in the point next south of 
Fort Doyle (by the weathered-out bands mentioned before). This 
vein is about a foot thick, and contains quartz, large plates of 
mica, and masses of orthoclase felspar as much as two or three 
inches in length. An extremely large quartz vein cuts across 
the Jerbourg peninsula nearly under the Doyle Column. There 
are many others of inferior size, especially in the gneiss ; and there 
are many networks of materials differing from the rocks they inter- 
sect, on whose origin it is difficult to be certain. 
Direction of the Dykes.—I made some notes of the general lie of 
the great greenstone masses which stripe the cliffs from Jerbourg to 
Pleinmont. They seem to run in certain prevalent general direc- 
tions. About Moulin Huet and Saints’ Bay they usually strike N. 
and dip from 30° to 60° E.; thence onwards to the Gouffre they strike 
W.N.W. or W., and dip from 45° to 60° N.N.E. or N.; however, a 
very large one at Moye Point strikes N.E. and dips 70° N.W.; thence — 
to the Creux Mahie the strike is W. and the dip N., and under 
Mont Herault. the strike is N.W. and the dip about 45° N.E. There 
are, of course, exceptions, but even the exceptions sometimes bear 
evident relations to the rule. I made these observations in hope of 
finding some clue to the folding or rolling of the strata. There 
