6 GEOLOGY OF THE SON VALLEY, ETC. 



All these facts point to the conclusion that the thin bands of 

 gneiss are in reality intrusive sheets, emanating 

 from the main granitic mass, which have taken 

 on a foliated structure like the thin sheets of the so-called " central 

 gneiss" in the Himalayas. The intercalation of the gneiss and 

 schists is in fact a case of what Professor Edgeworth David has 

 called " sill structure." 



Comparatively little attention was paid to the lithology of the 

 gneissose and granitic rocks. On account of the 



Lithology. . ."..,.. 



comparative rapidity of their decomposition 

 they occupy low ground, round which the harder rocks of the transi- 

 tions stand up in hills ; in consequence of this depression of the 

 surface it is largely covered with recent deposits, and the rocks very 

 ill exposed, except occasionally in the stream-beds. The rock is 

 generally a massive crystalline gneiss, in which the foliation is 

 often so indistinct that it becomes unrecognisable in hand-specimens. 

 At times the rock is porphyritic with orthoclase crystals running to 

 a couple of inches in length and scattered irregularly through the 

 rock with their longer axes at all angles with the foliation planes. 



The massive forms are usually found in the main exposures, and the 

 more foliated forms near their boundaries and in the thin sheets 

 penetrating the transitions. 



Associated with the gneiss are veins of aplite, composed of 

 quartz and pink felspar, which often becomes a graphic granite. 

 Quartz-veins are common, and there are numerous intrusive dykes 

 of dark-coloured aphanite and hornblendic trap which become 

 amphibolite in many cases ; unaltered cores of augite in the centre 

 of the hornblende crystals show this to be an altered pyroxenite. 



An intrusion worthy of special note is one of a coarse-grained 

 olivine-norite, in the bed of the Son near Kaithaha. It consists 

 of apatite, olivine, enstatite and augite, biotite, plagioclase, with 

 actinolite and enstatite, bictite and magnetite occurring as secondary 

 minerals. This rock which has been described by Mr. Holland 1 



1 Rec, Geol. Surv. Ind., Vol. XXX., pp. 20-21 (1397). 



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