138 Reports and Proceedings — 



but the stanniferous rock passes gradually into granite ; and they 

 show no signs of banded structure due to the successive deposition 

 of minerals. The highly granitic character of part of the South 

 Wendron tin deposit is strongly confirmatory of this view, which is 

 further supported by the occurrence, in the dark mass of the so- 

 called lode at the Lovell, of pseudomorphs of quartz after orthoclase 

 containing a little cassiterite. 



3. "On some of the Stock works of Cornwall." By Dr. C. Le 

 Neve Foster, B.A., F.G.S. 



The author commenced by explaining that the term '-'Stockwork " 

 had been derived from the German StockwercTc, meaning " Storey- 

 work," in allusion to the method of working in steps or storeys 

 in open workings originally adopted for such deposits. Their being 

 worked in open quarries affords a good opportunity of studying the 

 mode of occurrence of tin ; and many of them are interesting on 

 account of the small percentage of tin which will cover all expenses. 

 Thus, in the Wheal Prosper Mine, the average amount of oxide of 

 tin obtained per ton of stuff is not more than 3 lbs., worth, at the 

 present price of "black tin," 4:^d. per lb., so that the ground as it 

 stands is only worth I3^d. per ton. The mine can be worked 

 without loss on account of the softness of the rock and the large 

 size of the grains of tin-ore, the comparative lightness of the sub- 

 stances 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 inclosing 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 Rocks of Charnwood Forest." Part II. 

 By the Rev. E. Hill, F.G.S., Fellow and Tutor, and the Rev. Prof. 

 T. G. Bonney, F.G.S., Fellow and late Tutor of St. John's College, 

 Cambridge. 



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 ground mass of pulverized 



