38 PROr. T. G. BONNEY AND THE REV. E. HILL: [Feb. I9I2, 



sharply, but irregularly, jointed, so that it is difficult to obtain 

 specimens of a fair size and shape. 



In Guernsey they are nowhere common in the diorite, and some- 

 times altogether wanting, as on the shore north of St. Peter's Port. 

 One or two, however, may be seen in the Delancey-Lane quarry and 

 in the micaceous diorite on the south-western margin of Grand 

 Havre ; and they cut the granite in the south-eastern part of Lan- 

 cresse Bay, though none have been noticed in that of the eastern 

 horn of the Mont-Cuet promontory.^ In the gneissic region they 

 are abundant, as on the shore oj^posite to Lihou, at the foot of 

 Pleinmont, in Bon Kepos Bay, about La Corbiere, in Fermain Bay, 

 to the south of St. Peter's Port, and abundantly at Castle Cornet, 

 where they afford, as will presently be described, some special 

 features of interest. They have not been identified with certainty 

 in Sark,^ but they are far from rare in the diorite of Alderney, 

 being numerous at its south-western end,^ 



To obtain materials for a complete account of these dykes would be 

 so expensive a task that we must leave it (if it be worth undertaking) 

 to the officials of some future Geological Survey. We have, there- 

 fore, contented ourselves with examining under the microscope (in 

 addition to those from Castle Cornet, which can be more con- 

 veniently described separately) some dozen specimens (ten of them 

 new preparations), as illustrative of the more conspicuous varieties. 

 But as a minute description of even these would be wearisome, we 

 shall condense our notes into a general statement, only entering 

 into particulars in exceptional cases. 



The dominant minerals are quartz and felspar. Both are apt 

 to vary in size (in one or two cases, from about '08 to -0015 of an 

 inch), the smaller grains sometimes forming a sort of mosaic 

 setting for the larger,* or filling up a crack in them, while in other 

 instances the grains are more nearl}- of a size. The larger are apt 

 to have rather irregular boundaries, and occasionally are very 

 slightly elongated in one direction. The felspar is generally rather 

 turbid : but orthoclase, a plagioclase (perhaps oligoclase), and micro- 

 cline can be recognized. The last is commonly water-clear, though 

 containing a little (residual) dust. A third constituent is a more 

 or less chloritized biotite, generally small and infrequent, though 

 occasionally (as in a dyke on the north side of Fermain Bay) it is 

 large enough to give the rock a rather spotted aspect : but here also 

 it is considerably decomposed. Other minerals are practically 

 absent, except a few small grains of iron-oxide, to which, in an 

 ultra-microscopic condition, the frequent red colour must be due.^ 



^ That is, on the west side of Lancresse Bav ; see G-uernsey N. H. Soc. Trans. 

 1898, p. 255. 



2 E. Hill, Q. J. G. S. vol. sliii (1887) p. 332. There are doubtless many 

 reddish aplitic dykes in that island, but such evidence as can be obtained 

 seems to point to an earlier date for these. 



^ Near Fort Clonque one of them, as will be described, is cut by a later 

 dyke rather similar in composition. 



•^ For instance, a dyke about 24 feet thick, near Fort Tourgies in Alderney. 



5 To this group belong 20 & 21, in the appendix to Mr. Hill's paper of 1884, 

 Q. J. G. S. vol. xl, p. 423. 



