36 PROF. T. &. BOXNEY AXD THE BEY. E. HILL : [Feb. I912, 



We may, however, mention a variety from the north of the 

 causeway to Lihou Island, because it is the only instance in which 

 we remember to have found a true dyke (generally under 2 yards 

 in width) of this rock in the gneiss.^ A rather compact-looking 

 dull-green ground-mass is spotted with darker hornblende-grains, 

 rounded in form, and about a fifth or a quarter of an inch in 

 diameter. The slice shows them to be rather irregular in outline 

 and poecilitic in structure, enclosing a small mineral, not felspar. 

 Much of it may be some kind of augite, the remainder is a rounded 

 mineral ; in colour rather brown, with dark bands, possibly indi- 

 cating the former presence of an olivine. The ground-mass is in 

 still worse preservation ; hornblende, however, is rather abundant, 

 dominating the (much decomposed) felspar. 



These varieties, as well as the more normal granular diorite, 

 may be found among the specimens described in 188i, in which 

 the hornblende is commonly green, and the felspar (plagioclase) 

 too decomposed for further determination : sometimes the latter 

 is fairly idiomorphic, and sometimes a trace of fluxion-structure is 

 perceptible. 



Examples come from quarries west of Vale (the darker variety), 

 at Hommets (east of Cobo Bay), near Fort Doyle, in Delancey 

 Hill, the shore under Mont Crevelt, the northern end of Bellegreve 

 Bay, west of St. John's Church, at Castle Cornet, and Fermain 

 Bay. N'ow and then we find a little biotite or free quartz, espe- 

 cially in the slices from the more northern localities. 



Two specimens, from Grand Havre, may be mentioned as repre- 

 sentative of the most northern and acid type of the Guernsey 

 diorites, both being fairly coarse-grained. The one is from the 

 rocky shore of a promontory (near a tower) on the west side of 

 that inlet. A slice shows the felspar (in moderate preservation) to 

 be fairly idiomorphic, much of it being a plagioclase with rather 

 small extinction -angles, probably oligoclase, but some of it bears 

 more resemblance to an orthoclase. There is a fair amount of 

 hornblende and biotite closely associated : the former occasionally 

 including small crystals of felspar, the latter sometimes associated 

 witK its companion in a way that is suggestive of production at 

 its expense. There is a fair amount of quartz, which has been 

 the last to crystallize ; also a little iron-oxide and some small 

 apatites. The rock, in fact, is a tonalite. 



The other specimen represents the lighter rock in a quarry west 

 of Vale Church, and may also be called a tonalite. The hornblende 

 is rather irregular in outline, with poecilitic felspar in the larger 

 crystals ; the biotite is similar in shape and habit, and sometimes 

 contains rather minute inclusions of a green mineral which much 

 more resembles ill-preserved residual hornblende than a local 

 chloritization of the biotite. In fact, the two minerals are often 

 so related as to suggest that the latter has replaced the former 

 after the rock had become very nearly, if not wholly, solid. The 



' Mentioned by Mr. Hill, Q. J. G. S. vol. xl (1884) p. 417. 



