236 Geological Society. 



of the grains of tin-ore, the comparative lightness of the substances 

 associated with it, and the command of water-power. 



The deposits worked as stockworks occur in Cornwall in killas, 

 granite, and elvans. The tin-ore, associated with quartz and with 

 small quantities of other minerals, is found in more or less parallel 

 thin veins and strings, dipping at a high angle, and occasionally 

 giving off branches or uniting with one another both in dip and 

 strike. In the killas the rock close to the veins is occasionally 

 altered into tourmaline-schist; in the granite the walls of the veins, 

 and sometimes the whole mass of granite, are altered into greisen 

 and schorl rock. At Carclaze the orthoclase of the intervening bands 

 of granite has been converted into china-clay, which is now the main 

 object of the working. At Carrigan the leader sometimes adheres 

 to the enclosing rock by one side only, the other being bounded by 

 a clay vein which contains broken crystals of cassiterite, indicating, 

 in the author's opinion, that a movement of the walls has taken 

 place since the deposition of the tin-ore. Of the stockworks in 

 elvans the author gave a list, and remarked that the elvan of the 

 Terras Mine is particularly interesting, as it presents a series of 

 cavities left by the removal of orthoclase, and now being filled up 

 with schorl and a little oxide of tin. 



4. " The Precarboniferous Eocks of Charnwood Forest. — Part II." 

 By the Eev. E. Hill, P.G.S., Pellow and Tutor, and the Rev. T. G. 

 Bonney, F.G.S., Pellow and late Tutor of St. John's College, Cam- 

 bridge. 



The authors described the result of the microscopic examination of 

 a considerable series of the clastic rocks of Charnwood. Many of 

 these, even among the finer beds, prove to be of pyroclastic origin. 

 The coarser are generally composed of a groundmass of pulverized 

 felspar, with viridite and some iron peroxide, full of larger frag- 

 ments of felspar crystals (generally both of orthoclase and plagio- 

 clase) and lapilli. The structure of these is often distinct, some are 

 certainly andesites, others some kind of trachyte ; slaty fragments 

 are also present, and occasional grains of quartz. The authors 

 express their opinion that all the larger felspar crystals, and most, if 

 not all, the quartz grains, are of clastic origin, even in the more 

 highly altered varieties. Some of the larger fragments in the 

 breccias were examined, and referred in part to devitrified trachytes 

 not very rich in silica. The igneous rocks were then described. The 

 syenites of the southern and northern districts were shown probably 

 to belong to one system of intrusion. The hornblendic granite of 

 the Quornden district was also described, and the microscopic structure 

 of the different varieties of it and the above investigated. A number 

 of igneous rocks generally forming dykes in these was described : 

 some appear to be altered basalts, others andesites ; one is a felsite, 

 another a diorite. A group of outlying igneous rocks in the 

 vicinity of Narborough was described. Of these, one is a quartz 

 felsite with some hornblende ; another varies between this and a 



