294 



results may be produced by the metamorphosis of rocks of dissimilar origin. 

 We may mention the following as terms that have been applied to rocks 

 produced by the metamorphosis of acid igneous rocks ; but it must not 

 be supposed that all rocks to which these terms have been applied are 

 necessarily of similar origin : greisen, luxullianite, trowlesworthite, porphy- 

 roid, gneiss, granulite halleflinta mica-schist. 



Greisen. This term is applied to a rock essentially composed of 

 quartz and mica. Topaz occurs so frequently that it may almost be 

 regarded as an essential constituent. The rock is found in tin-stone 

 regions and appears to be a granite metamorphosed in connection with 

 exhalations of fluoric acid. 



Luxullianite. This term has been applied to a remarkable variety of 

 tourmaline-granite occurring in the form of large blocks near the village of 

 Luxullian in Cornwall. The rock has been described by Prof. BONNEY.W 

 It consists essentially of reddish orthoclase, tourmaline and quartz, and 

 may be regarded as a granite metamorphosed in connection with exhala- 

 tions of boracic and fluoric acids. 



Trowlesworthite. This name was introduced by Mr. WORTH. The 

 rock has been described by Prof. BoNNEY (2 >. It consists of orthoclase 

 tourmaline, fluor and a little quartz. The tiuor takes the place of the 

 quartz in the ordinary granite. 



Porpkyroid. This term has been applied to certain rocks which 

 are intimately associated with Palaeozoic sediments in Thuringia, the 

 Hartz, the Taunus and the Ardennes. They occur as lenticular masses and 

 as sheets and frequently exhibit a kind of passage into the surrounding 

 rocks. This apparent passage is often due to the fact that the marginal 

 portions of masses of porphyroid are markedly schistose ; the planes of 

 schistosity running parallel with those of the adjacent rock. Many of the 

 porphyroids are distinguished from the porphyries merely by the presence 

 of wavy planes occupied by sericite or some other micaceous mineral 

 (flaser- porphyroid). By an increase in the number of these planes and by a 

 corresponding increase in the micaceous mineral the rock passes over into 

 a schistose porphyroid and finally into a sericitic schist. 



The porphyritic constituents of the porphyroids are the same as 

 those of the porphyries, viz. : orthoclase, plagioclase and quartz. The 

 ground-mass may be massive or schistose. Hand specimens from the 

 central portions of a mass of porphyroid are often undistinguishable 

 from a quartz-porphyry whereas the marginal portions of the same 

 mass may exhibit a most marked schistosity. With the coming in of 

 schistosity the angles of the large porphyritic felspars become more or less 

 rounded off and the crystals themselves cracked at right angles to the 

 direction of stretching (3) . 



Porphyroids of the above type shade into hallefiinta-like rocks by the 



(1) Min. Mag., Vol. 1., p. 215. 



(2) Trans. Boy. Geol. Soc., Cornwall, 1884. 



(3) This direction is the longest axis of the strain-ellipsoid. It is clear that the sjliUt jja 

 porphyroids of the type above referred to are merely deformed masses of quartz-porphyry. 



