178 



ECONOMIC GEOLOGY 



an incrustation on an ingot of ancient tin, as a secondary crystal- 

 lization on reniform masses of wood tin. F. W. Clarke regards 

 all these as evidences or proof that the famous Cornish ores are of 

 aqueous origin. 



According to F. A. Genth, pseudomorphs of cassiterite after 

 hematite occur at Durango, Mexico. W. Semmons has also 

 described the presence of cassiterite as a concentric coating on 

 bismuthinite. 



According to Thomas and MacAlister, cassiterite occurs as an 

 original constituent of a few granites and acid volcanic rocks 

 diffused more or less uniformly through the rock mass, but par- 



FIG. 98. Vein of Cassiterite, quartz and tourmaline, traversing Paleozoic 

 slates which consist of alternate bands of siliceous and argillaceous materials, 

 Belowda Beacon, Cornwall, England. (After Thomas and MacAlisler's 

 Geology of Ore Deposits.) 



ticular as inclusions in mica. Such primary segregations they 

 regard as insufficiently large to be of direct economic value, but 

 they may ultimately yield rich alluvial deposits. 



The detrital deposits bearing tin are derived from the disinte- 

 gration and wearing away of the acid intrusives and contact 

 metamorphic rocks in which the tin lode occurs, and its presence 

 in the placer gravels at no considerable distance from its original 

 home to the transporting and sorting power of water. The ores 

 of tin appear to occur as primary segregations, as pneumatolytic 

 deposits, as metasomatic replacement deposits, and as detrital 

 deposits. 



