VEIN MINERALS. 



217 



Especial mention must be made of the peculiar tinstone from 

 Mr. Fitzherbert's Sinthe mine. It is found in the surface deposits 

 of a hill-side overlying decomposed argillites. The ore is really 

 a coarse, angular cassiterite but it is so rusted and penetrated by 

 oxides o iron, that in bulk it resembles heaps of coarse rusty sand. 

 When the surface soil is removed the underlying rock is seen to 

 be penetrated in alljlirectioiis with innumerable veinlets of a blanch- 

 ing system. The maximum thickness of these is 12 inches but 

 most of them are extremely thin and almost microscopic. One 

 veinlet was sectioned and proved to be J inch thick with clean 

 walls. Under the microscope it showed fine cassiterite grains at 

 the edge and a coarser grain inside. The centre of the veinlet 

 waa full of a brown and yellow iron mineral. A few laths of mica 

 also occurred, otherwise there was no other mineral present. The 

 matrix was a greenish-grey, vitrified argillite with irregular grains 

 of quartz. 



Cass'teiite has also been found in laterites and lateritic gravels 

 from various localities and is then masked by iron staining. 



J. Morrow Campbell has described another variety of cassiterite 

 obtained from Hermyingyi, Kalonta, and other places. He has 

 termed it the " prismatic " form, though the term is not too well 

 chosen, as it is liable to lead to confusion with the ordinary stout, 

 prismatic crystal forms. "It is found in small prismatic crystals 

 radiating from a central nucleus. They show aggregate polarization 

 between crossed nicols. The existence of this form of cassiterite 

 has not, as far as I am aware, been previously recorded. It has 

 two modes of occurrence. (1) in bands parallel and alternating 

 with quartz,— crustification — in veins, and (2) with the interspaces 

 of prisms and groups filled with .^oft micaceous mineral. The first 

 specimen of this I saw was picked up on an old alluvial dump 

 and though it weighed about a pound had been rejected as worth- 

 less by Chinamen. It is so fragile that streak or hardness 

 tests are difficult." 1 



The size of the grains of Tavoyan stream tin depends entirely 

 : . , . on the distance it has travelled from its ori- 



Alluvial cassiterite or . . , T . 



•' stream tin." g'nal home. In the upper parts of the valleys 



where little classification of the river deposits 

 has taken place, and where there is still much unsorted true detrital 



1 Morrow Campbell (9). p. 21. 



