38 HOLLAND: MICA DEPOSITS OF INDIA. 



The very frequent occurrence of valuable mica-bearing pegmatites 

 in association with mica-schist is a circumstance worthy of note : the 

 fact is important to the prospector and significant from the purely 

 scientific point of view. More than once it has been asserted that 

 in composition pegmatites vary with the rocks they traverse, and 

 it has sometimes been rashly concluded in consequence that the 

 minerals which compose the pegmatite have been derived by leaching 

 out of the country rock and deposition of the materials in fissures. 



The coarse-grained contemporaneous veins (pegmatites) which 

 traverse diorites, syenites and granites are probably end-products of 

 the differential consolidation of the magmas from which the associated 

 massive diorites, syenites and granites are derived. In such cases, 

 naturally, there is a mineralogical correspondence between the 

 pegmatite vein and the rock it traverses, a correspondence not difficult 

 to explain and one naturally to be expected. But as a matter of fact 

 the best mica-bearing pegmatites, though having the mineral com- 

 position of granite, are not generally found traversing that rock. On 

 the contrary, in the same area (take Ga\van in the Haz^ribagh district, 

 for instance), valuable mica-pegmatites will be found traversing the 

 mica-schists, whilst w-orthless veins occur close at hand in the massive 

 granite. That there is some essential connection between the mica- 

 bearing characters of the schist an.d the pegmatite seems evident, and 

 there are many facts to be observed in the mica-mining country which 

 remind the observer of Doelter's artificial formation of muscovite 

 by the action of potassium fluo-silicate and aluminic fluoride on 

 andalusite. 



We have found it necessary, in studying the physical conditions 

 attending the injection of pegmatite magmas, to accept a compromise 

 between the ultra-igneous theory and its antithesis. The writer con- 

 siders it necessary also to accept a modification of the theory that 

 mica-bearing pegmatites are the result of the simple consolidation of 

 an injected magma, and to allow that, whilst the pegmatites have 

 affected the surrounding schists, it seems likely that the schists in 

 return have modified the composition of the pegmatites, The com- 

 ( 28 ) 



