DELHI QUARTZITE SERIES. 77 



sinuous curves, the apparent boundaries of the quartzite are very 

 closely identical with the actual boundaries that would be laid 

 bare under the alluvium. There is no reason to think that the 

 subaerial agents of denudation (weathering, erosion, etc.) have 

 to any serious extent planed down a quartzite mass here, and 

 hollowed it out there, whilst leaving it elevated and continuous 

 elsewhere. And the chief evidence for this opinion is necessarily the 

 fact that no Delhi quartzite at all is found in the plain areas of Idar 

 State, whereas there should be at least some vestiges of it round the 

 present margins were it really continuous for any distance below the 

 alluvium, but with its edges truncated and ground down to the same 

 base-level of erosion as the Aravalli rocks themselves have been 

 reduced to. Such an instance is very distinctly seen in the area 

 south of Delhi town but not in this area. 



I conclude then that so far as this region is concerned, the 

 apparent outcrop areas are for all practical purposes the real areas 

 of the quartzite, and that we may therefore now study the lie and 

 trend of these masses in all confidence that they truly expose practi- 

 cally all the rocks of this nature that are left standing in relief 

 among the softer or more easily weathered masses of the Aravalli 

 schists, gneisses and pegmatites, on the one hand, and of the simi- 

 larly unresisting phyllites on the other. 



(2) Descriptive Details. 



Compared with the great variety of petrological type found 



_ __ . . among the Aravalli rocks, the Delhi quartzite 



Lack of variety. . . . ... 



series is monotonous m its uniformity. In 



saying that it is a quartzite, one has said nearly all there is to be 

 said, and it is merely necessary to mention a few lithological gene- 

 ralities as to the grain, colour and superinduced structures, before 

 going on to a detailed description of the special areas where the 

 quartzite is developed. We may sum up these as follows : 



The rock with the exception of some doubtful layers near Khercha. 

 _ „ , that will be referred to later is generally of 



Lithology. . . 



from moderate to coarse gram, the micro- 

 structure being generally that of a completely altered and recrys- 

 tallised quartz mosaic in which none of the outlines of the original 

 sedimentary material appear to have been preserved. (PI. 12. fig. 

 4 nat. light, fig. 5 crossed nicols.) It might sometimes be called a 



G 



