GEOLOGICAL OCCURRENCE. 35 



pegmatites, explains the high degree of fluidity and consequent injec- 

 tion to great distances of very thin films, as well as the remarkably 

 well crystallized condition in which such thin veins of pegmatite are 

 invariably found. 



It is to be expected, naturally, that the proportion of water to solid 

 matter in the material which forms pegmatite veins will not always be 

 the same, and, as a consequence, we get all stages between veins which 

 are practically formed from igneous magmas, and which show the 

 usual phenomena of more essentially igneous injections, and those 

 which contain an excess of water and consequently approach chemical 

 precipitations from solutions in fissures, The latter circumstance 

 accounts for the occasional "comby" structures found in pegmatitic 

 veins, whilst the former has given rise to the idea that pegmatites are 

 essentially igneous in origin. 



After a long period of sharp, not to say bitter, controversy between 

 two opposed schools, geologists are beginning to recognise the fact 

 that pegmatites are not to be relegated to either extreme explanation ; 

 but that the phenomena they present are due to a combination of both 

 igneous and aqueous agencies, a conclusion which involves no difficulty 

 since Guthrie and others have taught us to observe that igneous fusion 

 and aqueous solution are not separated by a sharp line of demarca- 

 tion, but, like many other natural phenomena, pass into one another 

 by insensible gradations. 



Form of pegmatite masses. 



In India, as in the mica-mining areas of America, the pegmatites 

 are found associated with mica-schists, quartzites and other schistose 



FlG. 8. Pegmatite dyke cutting through the schists and sending out apophyses 

 parallel to the folia (plan). E. of Gidhaur hill, Beh ir (after Mallet). 



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