PHONOLITE OF THE WOLF ROCK. 251 



crystals are enclosed in vitreous quartz, sometimes quartz lines each 

 side of the vein, or occupies the middle, or alternates with bands of 

 orthoclase, or orthoclase mixed with quartz, as in graphic granite. 

 Garnet and tourmaline are common as accidental minerals. Such 

 veins are distinguished by their shortness and irregularity. These 

 concretionary veins are frequent in the Laurentian rocks of Canada, 

 which are alternations of gneisses and limestones ; but they pass on 

 the one hand into metalliferous veins, and on the other into veins 

 with calcite, apatite, and various calcareous and magnesian silicates, 

 accompanied by orthoclase and quartz. Banded structure and drusy 

 cavities are characteristic evidences of deposition from water, though 

 the walls may be coated with hornblende, phlogopite, or other minerals 

 usually accounted igneous. 1 



Syenite of Ailsa Craig. 2 Ailsa Craig is an island ten miles west 

 of Girvan, 1113 feet high, 1500 yards long, and 1250 wide. It is a 

 fine-grained syenite, showing close parallel vertical joints on the south 

 and west sides. Dykes of dolerite 'run into it in a N. W. direction. 



Phonolite. 



The Wolf Kock lies nine miles S. W. of the Land's End, and at high 

 water is covered by the sea to a depth of two feet. The rock has a 

 yellowish grey base containing clear crystals of glassy felspar. Under 

 polarised light, crystals of felspar and nepheline are seen to be em- 

 bedded in a fine-grained matrix of nepheline, felspar, and hornblende. 

 In thick sections the felspar and nepheline are well-coloured * but in 

 thin sections colours are only shown by the hornblende, the hexagonal 

 sections being black and the rectangular sections white. The felspar 

 also has the aspect of a mosaic of dark and light stones ; it frequently 

 encloses crystals of nepheline and hornblende, and ' gives all ' the 

 characters of orthoclase ; it contains many glass cavities and minute 

 crystals. Hornblende is found in small green prisms sometimes 

 crowded about grains of magnetite. 3 



The greater part of the rock consists of nepheline, in crystals which 

 vary from T Jo^ * ToW^li of an inch across. This rock, probably of 

 Primary age, is classed by Mr. Allport as a porphyritic phonolite. 

 The following is the analysis by Mr. J. A. Phillips, F.K.S. : 



1 T. Sterry Hunt, Chemical and Geological Essays, 1875, P- J ^3- 



2 Mem. Geol. Surv. Scot. Explanation Sheet 7, 1869. Arch. Geikie. 



3 Allport, Geol. Mag., vol. viii. p. 247, 1871. 



