316 



A very remarkable variety of granite has been described by Professor 

 BONNET (i; under the name of Trowlesworthite. It appears to be a local 

 variety of the red granite of Trowles worthy and is composed of red 

 felspar, acicular tourmaline, purple fluor spar, and an occasional grain 

 of quartz. The fluor spar forms about one-fifth or one-sixth of the 

 entire mass and appears to occupy the place of the quartz of the 

 normal granite. The felspar is much decomposed and occasionally 

 contains patches of tourmaline and quartz. Where tourmaline has been 

 developed in the felspar of the granites now under consideration it is 

 almost, if not always, accompanied by quartz. In addition to the 

 accessory constituents already mentioned we find also in certain localities 

 cassiterite, pinite and andalusite. Mr. PHILLIPS mentions the occurrence 

 of garnet and beryl. Cassiterite occurs in the granite of Shaugh in 

 Devon according to Mr. WOKTH. Pinite in the form of cordierite is 

 found in the granite of the Land's End. Andalusite is present in the 

 rock which is quarried near the Cheesewring. It appears to be a 

 normal constituent of the granite. 



A remarkable variety of granite occurs at Meldon near Okehampton. 

 It is almost a pure white, somewhat resembling statuary marble in 

 appearance and is composed essentially of quartz, felspar (largely plagio- 

 clase), white mica and topaz. < 2 > Black mica is absent. The rock is 

 therefore a muscovite-granite. It contains also green tourmaline. 



The granite of Dartmoor is certainly of later date than the Culm- 

 measures and earlier than the Trias. The other granites above referred 

 to, including also the granite of the Scilly Isles, are presumably of the 

 same age. 



Granite occurs in the Lizard peninsula as veins in serpentine, 

 gabbro and a dioritic rock. It is, however, very different in character 

 from the granites already referred to. At Kynance it occurs as veins or 

 dykes in the serpentine ; at Pen Voose it may be seen to vein a 

 dioritic rock in the most intricate manner ; in a cove north of Pen 

 Voose it occurs in gabbro. At Kennack Cove it may be also seen 

 veining diorite. The granite of the Lizard, like the gabbro and serpentine, 

 has been profoundly modified locally by dynamic metamorphism, and 

 passes over into gneissose and schistose rocks. The least modified 

 granite is of medium or fine grain and is composed of quartz, felspar 

 and dark mica. It is generally pink in colour and often becomes a 



(1) Trans. Eoy. Geol. Soc., Cornwall. 1884. Vol. X., Part 6, p. 180. 



(2) The topaz in this rock rarely shows good form in thin sections. It is colourless and 

 possesses the characteristic refraction and double-refraction. To remove all doubt as to the 

 identification of the mineral the powder of the rock which passed through a sieve with fifty 

 meshes to the inch and was retained by one having eighty meshes was placed in a solution of 

 boro-tungstate of cadmium having a specific gravity of 3"2. The mineral in question fell down 

 whilst the other constituents floated. Cleavage flakes were easily picked out from the powder 

 and these gave in convergent light the interference figure of topaz they were seen to be at 

 right angles to a positive bisectrix and the optic axial angle was as near as could be judged that 

 of topaz. Grains of the powder pressed into lead could be made to scratch glass. Mixed with 

 microcomic salt and fused in an open tube acid vapours were given off which etched glass. 

 There can be no doubt therefore that the mineral is topaz although it occurs in allotriomorphic 

 grains and frequently does not show e~ood clcavace in the thin sections. 



