1917,] The fourth Indian Science Congress. cxcix 



the extremely natural and normal calcareous and other sediments show 

 no passage into anything resembling what are ordinarily known as crys- 

 talline limestones, calciphyres and calc-gneisses. 



Hence, failing any trace of a sedimentary origin, there have not 

 been wanting efforts to explain the limestone element of these rocks as 

 having been derived, by the action of carbonic acid in solution, from 

 pyroxenic ortho-gneisses, which by assumption are regarded as magmatie. 

 I need only refer to the classical work of Judd and Barrington Brown on 

 the Burma crystalline limestones (Phil. Trans. Roy. Soc, Vol. 187 A, 

 p. 205) and to the earlier interpretations of my colleague Dr. Fermor in 

 the case of the Central Provinces calciphyres (Rec. Geol. Surv. of India, 

 Vol. XXXIII, pp. 168-171). Quite recently, however, both my two col- 

 leagues in the Central Provinces, Dr. Fermor and Mr. Burton, have re- 

 turned to the more simple and straightforward conclusion that not only 

 the calc-gneisses but also the crystalline limestones have been derived 

 from a banded series of calcareous sediments of varying degrees of purity, 

 combined with lit-parlit injection of acid magma. This will be seen from 

 the quotation which I now reproduce from the General Report for the 

 Geol. Surv. of India for the year 1914-15. 



"Mr. Burton in his progress report, 1912-13, regards the crystalline 

 limestones as derived from sedimentary limestones of various degrees of 

 purity, and accepts the formation of mica, pyroxene, amphiboles, and 

 chondrodite, as due to the re-crystallisation of the original impurities in the 

 limestone, with pneumatoly tic addition of fluorine ; but the felspar in the 

 quartz-pyroxene gneisses he regards as in part of pneumatolytic origin. 

 He thus favours in the main the recrystailisation hypothesis. During the 

 past season's work (1913-U) Mr. Burton had the opportunity of devoting 

 further attention to these calcareous rocks as developed in the Balaghat 

 district. This led to an interesting development of ideas, so that whilst 

 Mr. Burton still supposes that the calc silicate minerals of the calc- 

 gneisses (calc-granulites) were in part derived from original impurities in 

 the calcareous sediments, he lays stress on the fact that the predominant 

 felspar is microcline with varying amounts of orthoclase, plagioclase being 

 present only in small amount or altogether absent. He deduces that thi> 

 microcline was derived from the associated orthogneisses during folding 

 when the latter became refused and attained the condition of an *&*&** 

 magma containing gases and pneumatolitic agents. The felspars both ot 

 the calc-gneiss and^of the ortho-gneiss show quartz inclusions {quartz as 

 corrosion), and this, Mr. Burton thinks, indicates that the calc-gneiss and 

 the ortho-gneiss must have crystallised under the same conditions ot pres- 

 sure, indicating that the calc-gneisses are reaUy mixed gneisses which 

 have re-crvstallised under plutonic conditions ' " 



In the rase of the Idar State examples, 1 may perhaps be allowed a 

 few words of description, since the observations and data are ot my oun 

 collecting, and have not been hitherto mentioned from this point of view. 



The rocks of this category, there, consist of erystalline limestone and 

 calc-gneiss, of various kinds within certain well-defined limits. I he 

 whole set is thoroughly well banded after the manner of any Arcnaean 

 schist or gneiss, and it is penetrated almost -every where by a very con- 

 stant set of lenticles, lenticular bands, branching dykes and occa.iona 1> 

 anastomosing granite aplite veins and occasionally by more massive in- 

 trusions of biotite-hornblende granite, known as the Idar granjte Itw 

 generally roughly equidimensional granular aggregate of frequently large 



' Daring the present field season (1914-15) Dr. Fermor has ac cepted 

 Mr. Burton's idea that these rooks are mixed gne.sses and both he n 

 Chhindwara) and Mr. Burton (in Balaghat) have arrived at i he co net won 

 that the hybridism has, at least in part been effected by the ht-p^ I «' in- 

 trusion of the calcareous rocks by an acid magma In Chhmduara. how 

 ever, labradorite is as abundant as microcline in the calc-gne.-se^. 



