106 MIDDLEMISS : THE GEOLOGY OF IDAU STATE. 



laboratory. The appearance under the microscope suggests a 

 metasomatic replacement of the quartz of the quartzite by the 

 manganese-bearing solutions, and the concomitant deposition of an 

 intimate plexus of veins and veinlets of secondary quartz and 

 chalcedony. In some cases large veins and rounded hollows, an 

 inch across, are filled with soft, sooty pyrolusite which lies loose 

 or falls out on breaking the rock, like a nut from its shell, as 

 ii) specimen No. 4 5 4 \. Other specimens of vein rock, generally 

 silicious and ferruginous are 4 2 4 ? 4 — 4 2 4 V of these, 4 \% shows in a 

 quartzite matrix long lath-shaped crystals, much altered, of (?) 

 a variety of amphibole. At the Vartha end of the band no thick- 

 beds of steatite or dolomite are visible, but minute flakes can fre- 

 quently be seen in the microscopic preparations. A little further 

 to the south-east, however, about 1J miles S.E. of Vartha, steatite 

 with chlorite beds, is present in considerable beds (specimens Nos. 

 2 9. — SLSL) *jk e occurrence being similar to that at Ghanta. 



447448/' w 



It will be seen that the evidence now accumulating concerning 



„ , . , , , ... these various occurrences of magnesian 

 Probably advent it 1- _ ... 



ous nature of the rocks, with their other associated minerals, 

 magnesian rocks. seems to a large degree to discount their first 



appearance of being original sediment a ry deposits interbedded 

 with the Delhi Quartzite. The apparent horizon of the rocks 

 is too near the uppermost limit of the Delhi Quartzite for them 

 to belong to some different and basal phase of the Delhi 

 Quartzite, such as the Raialo Limestone of N.E. Rajputana, 

 recently re-described by Mr. A. M. Heron {Mem., Geol. Surv. 

 India, Vol. XLV, ft. 1), and it is equally unlikely that they belong to 

 some rapidly thinning, local, sedimentary deposit at any higher 

 horizon. These magnesian rocks from many points of view, seem 

 to be really adventitious in some way ; to have become inserted 

 among the quartzites, and, either bulged them up into local anti- 

 clines, or wedged apart neighbouring quartzite layers. We might 

 imagine them as injected or deposited as veins or forced up along 

 gapes among the quartzite strata by some strong deformatory 

 movements such as elsewhere have resulted in stoping away the 

 Delhi Quartzite in blocks. Or on the other hand it may be that the 

 whole phenomenon is more of the nature of a normal peridotite 

 intrusion, accompanied by the usual secondary changes of such 

 rocks to serpentine, talc, magnesite and dolomite, and later still 

 affected metasomatically by mineralizing solutions carrying manganese, 



