aravatj.t system. 45 



body of the calc-gneiss — imagining the latter as an outer and 

 very wide aureole of more moderate metamorphism — is however 

 considerably uncertain. As in the case of the aplite veins, there 

 may be vast unseen developments of this granite hidden from view 

 by alluvium and other surface rocks. 



This exhausts all the available contact thermal action of well- 

 defined igneous bodies that we can appeal to in this area to account 

 for the mineralisation of the calc-gneiss ; and there only remains, 

 as possibly coming under suspicion, the biotite-gneiss. 



The question whether the biotite-gneiss might be regarded as 



an ancient orthogneiss that induced this meta- 



Possible action of the ,,,^..^i,;<.^-. n ± t.\>. „ i.- n •, , 



biotdte-gneias. morphism at the same time that it stoped 



and assimilated xenoliths of the Delhi quart- 



zite, may well lie given a short consideration. One difficulty here 



is that the biotite-gneiss is not an ordinary granite gneiss. 



The felspar, which is in small proportion, is a lime-soda plagioclase, 



whilst the enormous percentage of quartz is much above that of 



any ordinary igneous rock. But even here again it should be noted 



that on Daly's eclectic theory of igneous rocks 1 (whereby granites 



and other acid and intermediate plutonics are regarded as mixtures 



of an underlying fluid basic shell, reacting on and absorbing its 



acid and sedimentary crust rock-cover, the resulting " svntectic " 



subsequently crystallising out into various magmatic differentiates) 



one might regard a rock such as my biotite-ffneiss as being a srrano- 



diorite unduly acidified by absorption of the material of the Delhi 



quartzite, as indicated by its sloped contact with the latter. 



One cannot, then, dismiss this explanation as being entirely 

 inadequate, although there is no positive field evidence that would 

 in any way support the idea of the biotite-gneiss being a foreign 

 eruptive rock among the calc-gneiss. with which, on the contrary, 

 it shows a parallelism of foliation and of general aspect, as if it 

 had arisen in some similar way. 



There still remains the explanation that the calc-gneiss and the 



biotite-gneiss have both alike been derived 

 Regional metainor- f„~^ „ • j.- j- i 



p hisn i_ irom a pre-existing sedimentary series by sus- 



tained and perhaps periodic regional meta- 

 morphism (dynamo-metamorphism). It seems to me quite admissible 

 that dynamo-metamorphisin. resulting in rock deformation so intense 

 as to produce rock-flow, might have brought about all the mineral 



1 " Igncoua Rocks and their Origin " by R. A. Daly, Xcw York, 1914. 



E 



