ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 69 



the West of England are much the same as in other tin- 

 producing countries.* In the following lists of minerals found 

 associated with cassiterite in the different kinds of country rock 

 in the West of England, I only refer (a) to those which appear 

 to have been formed contemporaneously with the cassiterite, 

 and {b) to those which have been observed in immediate contact 

 with it and mostly deposited on it. 



Granite. — Here the most usual associates as thus defined 

 are — in the a class, besides the constantly occurring quariz, 

 felspar, (ortboclase, usually more or less kaolinized) mica (lepido- 

 lite, lepidomelane, and Gilbertite), and tourmaline', less frequently 

 wolfram and stannite ; occasionally topa%, apatite, scheelite, fluellite, 

 Tavistockite, Churchite, and molybdenite. In the h class we have, 

 quari%, mispickel, pyrites, chalcopyrite, smaltite, blende, bismuth, 

 bismuthenite, hematite, and limonite, with, occasionally, native 

 copper, pitch-blende, and uraniam- ochre. 



Elvan (Quartz and felspar-porphyries.)— Here in the a 

 class, quartz, felspar, tourmaline, and mica as in the granite ; in 

 the b class, quartz and hematite. 



KiLLAs (clay-slate, tourmaline-schist, "knoten-schiefer," &c.) 

 Cassiterite occurring in these rocks is almost invariably associ- 

 ated with quartz, and microscopic or macroscopic tourmaline. In 

 the b class we often &nd chalcedony (Pednandrea, East Pool, &c.) 

 chlorite, mispickel, pyrites and wolfram ; more rarely chalcopyrite, 

 smaltite, cobaltite, hematite, limonite, blende, chalybite, and Gilbertite ; 

 still more rarely, pitchblende, pearl spar, pharmacosiderite, uranochre, 

 chalcosiderite, llenwoodite, Andrewsite, olivenite, topaz, and apatite. 



HoRNBiiENDic Slates (pseudo-groenstones.) Tin is rare in 

 such rocks, but when it does occur, it is associated with horn- 

 blende, actinolite, apatite, epidote, axinite, and garnet, of the a class • 

 and magnetite, hematite, chalcocite, chalcopyrite, diallogite, Penwith- 

 ite, carbonate of bismuth, calcite. Dolomite, olivenite, scorodite, and 

 pharmacosiderite of the b class. 



Greenstone (Diorite.) In this rock the occurrence of tin is 

 very rare, but occasional veins occur associated with chlorite, 

 limonite, and other products of change. 



* e.g. Saxony, Bohemia, Brittany, Spain, Banca, New South Wales, &c. &c. 



