1917.] The Fourth Indian Science Congress. cci 



their bases. They are frequently extremely sheared and broken up into 

 platy layers near any Aravalli junction, against which they frequently 

 plunge with dips at right angles to the strike of the juxtaposed Aravallis. 

 The whole aspect of the Delhi Quartzite in this area is of enormous ridges, 

 sections and blocks of quartzite strata floundering about as it were in a 

 viscous Aravalli sea. Furthermore there are in certain places actual junc- 

 tions shown, where stoping of Delhi Quartzite blocks of from one foot or 

 less to several yards in width can be seen as it were in actual operation, 

 and where the whole of these blocks and many also of the half disintegra- 

 ted basal layers of the Delhi Quartzite have begun to get detached and 

 ready to break away under the stoping operations of the underlying 

 biotite gneiss And all of these blocks and loosened lower strata, it is im- 

 portant to notice, are in their interstices crowded with many of the typi- 

 cal cale-gneiss minerals, e.g. wollastonite, calcite, diopside and grossular 

 garnet — these contact-developed minerals weathering out at the surface 

 of the blocks and leaving a spongy, cavernous layer behind. Chaotic and 

 irregular as is the junction between these two systems in Idar, I am not 

 sure that the parallel arrangement in Ajmer of the same Delhi quartzites 

 to the underlying calc-gneisses (chiefly amphibole epidote gneiss there) 

 does not eclipse it. Last cold weather in the company of my colleague 

 Mr. Heron I had the privilege of seeing near Sendra some of the most 

 amazing relationships between these two formations, which, however, 

 would require diagrams to illustrate them satisfactorily and for which I 

 fear I have not time to trouble you with now, as we have a large number 

 of papers to get through this morning. 



Before concluding, I should say that the impressions I have gathered 

 in Idar with regard to these and other phenomena are on the whole in 

 favour of calling largely on the processes known as plastic deformation 

 and dynamometamorphisra for an explanation of very many of these 

 peculiar results. I am inclined to picture to myself that below a certain 

 level underground, such as may well have now been exposed by uplift 

 and denudation in the roots of this old Aravalli mountain chain, processes 

 of the above category may well have brought about in the Delhi Quartz- 

 ite, in the Aravallis below, or along the junction line between them, very 

 great and powerful metamorphosing actions that have simulated magma 

 tic stoping and assimilation, that have probably reduced much rock in 

 certain areas to the condition of aqueo-igneously fused rock-pulp, and 

 that have culminated in the production of gneisses. It is even allowable 

 to suppose that they may also have caused the inception among them of 

 intercrustal veins, lenticles and dykes of aplite and other pegmatitic vari- 

 ants, after the manner advocated by A. C. Lane, quoted in Daly's M Igne- 

 ous Rocks and theii Origin," p. 370, and named by him " selective solu- 

 tion." In that reference it is said that: ^During intense regional, 

 metamorphism, specially of the dynamic kind, deep-seated rocks, charged 

 with much interstitial water, may reach the relatively low temperature 

 at which minerals corresponding to the quartz- felspar eutectic go into 

 solution with the water and other volatile fluxes. Such small, locally 

 generated pockets, lenses or tongues of fluid may be driven through the 

 solid country rock for an indefinite distance ; subsequently to crystalline 

 with the composition and habit of the true b&tholithic derivatives. It is 

 thus quite possible that these particular rocks, though truly magmatic, 

 have had no direct connection with abyssal injections." 



In other words, it seems to me that from what may once have been 

 two unconformable systems, such as the Delhis above and the Aravallis 

 below, one may have had generated by plastic deformation and dynamic 

 metamorphism the appearance of a complete eruptive unconformity, sepa- 

 rating a lower gneissic from an upper quartzite series. And if that be 

 granted, then it is obvious that the great march of events here in these 

 northern rock areas can only be understood as being the exact reverse of 

 that which the Mysore Geological Department are advocating in the case 

 of the not dissimilar position in Southern India. 



