32 HOLLAND: MICA DEPOSITS OF INDIA. 



Cassiterite. Quartz, pink and white. 



Columbite, Staurolite. 



Epidote. Tourmaline, red, blue, 



Fluor-spar. green and black. 



Garnet. Torbernite. 



Ilmenite. Triplite. 



Kyanite. Uranium ochre. 



The large size of the crystals, facilitating their extraction, makes 

 some of these minerals, like the phosphates and felspar, worth attention 

 from an economic point of view, whilst the most valuable constituent 

 of all, mica, is of value purely because of the large size of the sheets it 

 forms. Crystals or " books " of muscovite-mica have been obtained 

 in Nellore district, measuring 10 feet across the basal planes, but 

 usually, of course, they are much smaller, all gradations of size being 

 obtained from those of marketable value down to scales of microscopic 

 dimensions such as occur in the common massive granites. 



Being the most delicate mineral in the rock the mica is the first to 

 show the effects of crushing earth- movements, and large quantities of 

 valuable mineral have by these means been destroyed ; but it is on 

 account of the remarkable stability of the Indian Peninsula, the 

 geologically long and perfect quiescence it has enjoyed, that India is 

 able to boast of the finest mica deposits of the world. 



Whilst the Himalayan range is composed of rocks which have been 

 crumpled and sheared even since late tertiary times, the Peninsula of 

 India has remained as a firm solid mass since at least the lower 

 palaeozoic age, and as a result many very old rocks, like the pegmatites 

 have been preserved with remarkable freshness. The main mica-bear- 

 ing area of the United States — the only area which has ever been a 



serious competitor with India for the first place in the mica market 



owes its value to a similar cause : the crystalline mass with its included 

 pegmatites, stretching from Georgia obliquely through the Carolinas 

 to Virginia, presents to the contorted Appalachian range exactly the 

 same relation as the Indian peninsula holds with regard to the 



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