HOENBLENDE-SCHISTS, GNEISSES, ETC. OF SAEK. 139 



Part II. The later Intrusive Eocks. 



Among the gneissoid rooks mentioned above it is not unusual to 

 find intrusive veins which are very closely welded to, and sometimes 

 seem almost inseparable from, the rocks into which they break. They 

 are generally moderately coarse, mottled reddish and greenish 

 granitic or dioritic rocks, with a superficial resemblance to the 

 basement-gneiss (being often slightly foliated). 



In some cases, as stated above (p. 134), they probably are that rock, 

 in others they are less quartzose. One from the cliffs south of Port 

 du Moulin is a mica-diorite, rather rich in apatite, and containing a 

 little quartz. So is another from Saignie Bay, but it contains one 

 or two zircons. Another from Greve de la Yille is much the same, 

 but with rather more quartz, and more nearly allied to the granite. 

 A fourth from Dixcart Bay, which breaks into a hornblende-schist, 

 is not very different, only it contains hornblende as well as biotite. 

 These, however, probably are not much more recent than the rocks 

 in which they occur. So we pass on to those intrusive masses 

 which are distinctly of a later date. These have been already 

 briefly described by Mr. Hill,' but we are enabled to add a few 

 particulars which may be of interest. 



First, as regards the great masses of granitic rock whicl form the 

 northern and the southern parts of the island. Undoubtedly, as 

 affirmed by Mr. Hill, these are intrusive into the group of hornblende- 

 schists and bedded gneisses; of this we obtained further proofs. 



It is, however, doubtful whether the southern mass should continue 

 to be designated a granite. It became evident on closer stud}?- in 

 the field that quartz was not conspicuously present, and microscopic 

 examination shows the rock to be a diorite, containing a fair pro- 

 portion of biotite, quartz occasionally occurring as an accessory. 

 Apatite is rather conspicuously present, but we do not purpose to 

 enter into the minor petrographical details. The Brecqhou rock 

 also appears to be rather a quartziferous diorite than a true granite. 

 There is rather more quartz and biotite in the rook at the northern 

 end of Sark, so it may be left with the hornblendic granites until it 

 has been submitted to a more minute petrographical study, but it is 

 not unlikely that it will prove to be, more strictly speaking, a 

 tonalite. 



Mr. Hill drew attention to the rarity of dykes of f elstone or fine- 

 grained granite. To the dyke of the Boutiques caves mentioned by 

 him (which cannot be distinguished from that on the headland) we 

 may add a rather similar, but slightly more compact-looking rock, 

 forming a dyke near the Port on the south side of Brecqhou, another 

 narrow compact dyke on the eastern end of Point Derrible, and a 

 third on the Burons which is a little darker in colour. 



The Boutiques dyke, on microscopic examination, exhibits some 

 rather small felspars, crowded with fibrous secondary microliths, and 

 giving bright tints with the crossed nicols, set in a mosaic of 

 quartz and felspar, the structure of which sometimes approaches to 



1 Quart. Journ. Geol. Soc. vol. xliii. (1887) p. 332. 



