ARAVALLI SYSTEM. 47 



at the 12th International Geological Congress at Toronto, where 

 the subject was discussed. The most recent pronouncement of 

 opinion of this Department (one may .say) is that of Dr. Fermor 

 and the late Mr. E. C. Burton, as expressed in the annual report 

 referred to above which was based on progress reports for the year 

 11)12-1913. Quoting from this:— 



Mr. Burton regards the crystalline limestones as derived from sedimentary 

 limestones of various degrees of purity, and accepts the formation of mica, pj-- 

 roxene, amphiboles. and ehondrodite, as due to the reerystallisation of the original 

 impurities in the limestone, with pneumatolitie addition of fluorine; hut the felspar 

 in the quartz-pyroxene gneisses he regards as in part of pneumatolitie origin. He 

 thus favours in the main the recrystallisat ion hypothesis. Daring the past season's 

 work ( 1913-14) Mr. Burton had the opportunity of devoting further attention to 

 these calcareous rocks as developed in the Balaghal district. This led to an interest- 

 ing development of ideas, so that whilst Mr. Burton still supposes that the eale- 

 silicate minerals of the calc-gncisses (calc-granulites) were in part derived from 

 original impurities in the calcareous sediments, he lavs 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 this 

 microcline was derived from the associated ortho-gneisses during folding, when 

 the latter became re-fused and attained the condition of an igneous magma contain- 

 ing gases and pneumatolitie agents. The felspars both of the calc-gneiss and of the 

 ortho-gneiss show quartz inclusions {(/nartz de corrosion), and this, Mr. Burton thinks, 

 indicates that the calc-gneiss and the ortho-gneiss must have crystallised under the 

 same conditions of pressure, indicating that the calc-gneisses are really mixed 

 gneisses which have reorystallised under plutonic conditions. 



A footnote to the above quotation states that " during the 

 present field-season (1914-15) Dr. Fermor has accepted Mr. Burton's 

 idea that these rocks are mixed gneisses and both he (in Chhind- 

 wara) and Mr. Burton (in Balaghat) have arrived at the conclusion 

 that the hybridism has, at least in part, been effected by the lit- 

 par-Jdt intrusion of the calcareous rocks by an acid magma." 



It is of considerable importance to the subject of pedogenesis 



generally to note that an acceptance of ex- 



oJnon 1 ° f m ° dern lunations on the above lines for the origin 



of the calc- and associated gneisses, and 

 perhaps for the vein rocks too, commits one to the view that the 

 distinction between para- and ortho-gneiss, and, bv implication, 

 that between sedimentaries and igneous rocks (owing to mutual 

 assimilation or hybridism), can no longer be regarded as a hard- 

 and-fast distinction : that the barrier breaks down, in fact, under 

 sufficiently intense conditions. The more conservative view, that 

 magmatie differentiation from more or less separate or connected 



e2 



