Vol. 68.] PETROLOGICAL NOTES ON GUERNSEY, ETC. 39 



IV. Rocks at Castle Cornet. 



This fortress, now occupied as military barracks, stands on a 

 rocky island, joining the southern pier of the harbour at St. Peter's 

 Port. The rocks between the walls and the sea are easily traversed 

 at low water and afford excellent sections. Gneiss, of which this 

 is the northern extremity, is the oldest rock. Diorite is intrusive 

 in this, acid dykes of a reddish colour cut both, and dark basic 

 dykes traverse all three. Without a large-scale map it would be 

 impossible to depict and tedious to enumerate the numerous and 

 complicated outcrops of these rocks, so we shall be content to 

 draw attention to a few points of special interest. 



The gneiss is reddish in colour; it varies slightly in coarse- 

 ness, the augen-structure not being always equally conspicuous, 

 and the cleavage-foliation is more or less soldered up near the con- 

 tact with an intrusive rock. 



The diorite is usually composed mainly of about equal quantities 

 of dark-green hornblende and felspar (neither well preserved), in 

 slightly irregular grains measuring about an eighth of an inch 

 in diameter ; but this common variety assumes over a limited area 

 the ' birdseye ' character.^ At a junction-surface the diorite is either 

 closely welded to the gneiss, or occasionally has partly melted 

 it down. In the latter case the diorite, for a few inches, assumes 

 a slightly streaky and porphyritic structure, being studded with 

 felspar-crystals (rather rounded in outline) which increase in size 

 as they approach the gneiss. They are, however, always distinctly 

 less than the larger ' augen ' in that rock, so that we are justified in 

 inferring the latter to have beeu partly, and the smaller consti- 

 tuents wholly, melted down. The larger felspars may be traced for 

 perhaps 4 or 5 inches from the indubitable gneiss, and the smaller to 

 quite double that distance. The diorite occasionally includes a strip 

 of gneiss, 4 or 5 inches thick, which may also have a fused border. 



In a slice from about the middle of a ' fusion-band ' the rock 

 presents a rather fragmental aspect. The principal constituents 

 are quartz, felspar, horubleude, and biotite, which are irregular in 

 form and variable in size. The quartz, although fairly abundant, 

 generally occurs in rather small grains ; the felspars range down- 

 wards from about -25 inch. The larger are sometimes fractured, the 

 crack being filled by minute ground-mass, l^one are well preserved, 

 but a plagioclase and some orthoclase can be recognized. The 

 hornblende and the biotite (the last altered to a pale-green mineral, 

 with extrusion of iron-oxide) are generally smaller in size than m 

 the ordinary diorite or gneiss, and are curiously associated, some- 

 times almost intermixed.^ Thus the inference from field-work is 

 confirmed by the microscope. 



The numerous red dykes often indicate no small shattering 

 of the mass composed of the two older rocks. A very few of these 



^ It is in the middle of a slight recess in the rock on the west side, near its 

 northern end. 



■^ There is also some iron-oxide and a little apatite. 



