gneiss 



Quart y.ii 



and 



i) ( 



■11, 



Delhi 



Quai 



tzite 



to 



here, 



and 



also 



to 



A RAVALLI SYSTEM. 26 



the composition near Ab 1 An ls or coming between the andesine and 

 [abradorite groups. They are also confirmed by the birefringence, 



which gives interference tints of grey and pale yellow, as compared 

 with the purple, bine and yellow of the associated quartz. The 

 refractive index as indicated by the movement of the Beoke line 

 is greater than that of the Canada balsam. 



There is a very small amount of calcite and some minnte zircons 

 in the slide. No iron ores except a little in the biotite. 



There is no garnet and no diopside in this specimen, these minerals 

 elsewhere being fairly common in rocks of this formation. Other 

 specimens from near Walren are fj' 9 and 4 ^&. 



The biotite-gneiss of the Walren-Dijio area, whose mineral corn- 

 junction of biotite- position bas i ,lst been under description, 

 becomes of special interest when considered 

 in its tectonic relations to the outcrops of the 

 the east. Very fortunately, the country just 

 the north of Derol, gives actual examples of 

 this junction in several clearly seen exposures. It is advisable to 

 mention these now, and before the Delhi Qnartzite itself has 

 been described, because of their immediate interest and the assist- 

 ance they give in eomprehendi og the not very simple role played 

 by the biotite-gneiss. If we follow the boundary in a southerly 

 direction from the Walren-Talan main road at a point g mile 

 W. by N. of the 1,150-ft. hill, it is not difficult to trace it continu- 

 ously in spite of the low jungle and involved hilly slopes. The 

 Delhi Qnartzite appears as composing all the spurs ascending up 

 to the 1,150' hill, and can be ascertained to be really in situ, though 

 frequently and generally much shattered and without definite dips. 

 Round the base of these spurs, where the angle of slope eases oil 

 into the lower-lying country, the well defined and "slabby" fol- 

 iation dip-faces of the biotite-gneiss wind their way with great 

 regularity, occupying most of the lower extremities of the spurs. 

 The line of junction is generally complicated by intrusive vein 

 granite (biotite-bearing aplite), but the point of chief importance 

 is that it wanders along, sometimes V-ing up stream-beds and at 

 other times behaving quite irrationally, but always as if a floor 

 of the biotite-gneiss were unevenly undulating under the quartzite. 

 In tracing this continuously to the river-bed east of Dijio, I was 

 at first struck by the appearance of transgression of the overlving 

 Delhi Quartzite series across the lines of foliation of the biotite- 



