USEFUL METALS 



188 



origin. In one case it implies the kaolinized pegmatites bearing 

 some quartz. In the other it signifies a true <! -trit.-il deposit. 

 In Parak there occurs the Lahat pipe which consists of an irn-Ku- 

 lar pipe-like body of cassiterite extending to a depth of several 

 hundred feet. The pipe was originally a tin-bearing vein but 

 subsequently became a course for surface waters. The adjacent 

 limestone was taken into solution and transported elsewhere. 

 The cassiterite fell into this solution cavity and was subsequently 

 cemented into a somewhat brecciated mass. 



According to Scrivenor, the detrital cassiterite deposits on 

 the islands of Banka and Billiton occur in two different ways. 

 In one case the tin is found on the hillsides and plains not far from 



FIG. 101. Reduction works of the Llallagua tin mine in Bolivia. 



the lodes from which it was derived. In the other it is found in 

 true valley detrital material which sometimes reach a depth of 

 50 ft. The tin-bearing portion of the placer lies in the lower 3 ft. 

 of the deposit and is associated with topaz and tungsten minerals. 



More than half of the world's supply of tin has been derived 

 from the Federated Malay States and the islands of Banka and 

 Billiton off Sumatra. The detrital deposits have been larger 

 producers than the lodes. 



A third foreign area of somewhat less importance is found in 

 Bolivia. The deposits are of special interest because the ores 

 are associated with lead, silver and bismuth minerals in a dacite 



