G MIDDLEM1SS : THE CiEOLOf.Y OF IDAR STATE. 



lying far ofE this line of strike, with his original Aravalli ; and so, 

 to some extent, depreciated the value of the term. Nevertheless, 

 since some name is necessary for these rocks, I think we are justified 

 in taking that which seems most properly to have belonged to them 

 in the first instance, or very nearly the first instance, since we 

 naturally accept an early modification of the term as matured by 

 Racket himself whereby the Delhi Quartzite is removed from the 

 Aravalli system and elevated into an independent higher system 

 (a modification the soundness of which has stood the test of time). 

 In other words the Aravallis here are. in the main, Market's Ara- 

 vallis as matured by him, with sundry exceptions found necessary 

 by later exploration. For further remarks on the present status 

 of the term "Aravalli" the recent memoir by Mr. A. M. Heron 

 on the Geology of Western Rajputana [Mem. (r. 8. of /.. Vol. XLV. 

 pt. 1) may be consulted. 



Under the title of Aravalli, then, comes all that assemblage of 



. , .. mineralogically and petrologicallv interest- 



A mixed assemblage . ° J * i ' 



of foliated crystalline mg components of the Archaean crystalline 



rocks - complex that are discoverable in this region 



as forming the lowest accessible portion of the earth's crust there 

 exposed. They are grouped together, and only partially distin- 

 guished by separate colouring on the map, mainly for that reason 

 most cogent to the field geologist ; namely, the impossibility of 

 alwavs separately delineating their individual boundaries and not 

 necessarilv because they are imagined as having had a single mode 

 of origin. Although few obvious signs of any sedimentary origin 

 obtrude themselves in their now highly altered condition, it is 

 reasonable to suppose that several of their members were originally 

 of the nature of stratified, or chemically precipitated, sediments of 

 some kind (paragneiss) ; whereas others appear to bear the mineral 

 constitution of igneous gneissic material. Hence although one might 

 theoretically draw hard and fast lines between much that is of one 

 order and of the other, still, in the practical business of geological 

 mapping such distinctions are here found to be impossible, because : — 

 (1) there are always some phases of the one or of the other that 

 might be regarded indifferently as either of igneous origin or of 

 metamorphic sedimentary origin, (2) these phases are often so 

 intermingled (kneaded together as one might say) that no map 

 of any available scale could accommodate all their boundaries, and 

 (3) nearly all lie so close to the base-level of erosion for this part 



