7C MIPDLEIUiSS: THE GEOLOGY OF IDAR STATE. 



respect it differs markedly from the similar quartzites of Alwar, 

 where distinct unconformity, with a basal conglomerate, is plainly 

 seen. Almost everywhere in Idar the self-contained Delhi Quartzite 

 builds connected or isolated ranges, or in some cases isolated and 

 small undulations and hillocks, bathed all round at their bases 

 by a sea of alluvium ; although this is not by any means every- 

 where a thick deposit. The Aravalli rocks on the other hand have 

 been already described by me as composing either low, undulating 

 country, surrounded also by alluvium, or as detached and restricted 

 outcrops in the beds of streams, likewise surrounded and covered 

 by Recent deposits. The Delhi Quartzite itself never appears at 

 very low levels exposed in the bods of the larger streams, and only 

 very occasionally in minor gorges which cut through, and directly 

 across, a prominent strike-ridge of quartzite. Thus the secret of 

 its direct and detailed relationship to the Aravallis is usually veiled 

 from actual view, and such conclusions as we arrive at concerning 

 it must be more or less reasoned conclusions drawn from scanty 

 data. 



Tn some cases no doubt the cause of this is the fair thickness 



to which the alluvium and wind-borne sand 

 to aita-num? 6 ** " attam ' l)llt tms ' s not everywhere the case ; 



and frequently there is no apparent reason 

 why, over so large a range of country, chance should have afforded 

 so few actual Contacts, either by portions of the Aravalli rocks 

 having been left protected in hilly form under cover of some quartzite 

 hill or spur, or by the same thing happening in one or other of the 

 river-bed sections and the quartzite being left in superposed contact 

 with the Aravallis. 



Another preliminary matter that requires mentioning is the 



Quartzite outcrops following : if we look at the map and 



and thoir actual dis- note the irresponsible way in which the 



outcrops of the Delhi Quartzite very fre- 

 quently wander, we feel the necessity of asking whether the 

 surface area coloured as Delhi Quartzite substantially coincides 

 with what the outcrops of it would be were the region stripped of 

 its mantle of alluvium. In other words, is the alluvium in any 

 effective degree responsible for these great irregularities ? I 

 believe, myself, that the apparent and real areas of quartzite do 

 practically coincide. I believe that when a quartzite ridge comes 

 to a sudden end against alluvium, or when it twists about in 



