PETROGRAPHICAt. NOTES. 6j 



©us to the unaided eye. It is frequently and characteristically 

 striped with bands of varying width of a bright-red colour caused 

 by minute particles of hematite. Examined with a high power of 

 the microscope, these hematite particles are seen to be scattered 

 through a very minute and regular mosaic of quartz or chalcedony. 

 They appear by transmitted light of a deep blood-red colour and 

 exhibit perfect crystalline outlines. Larger grains of quartz are 

 visible only as secondary infillings of cracks. Sometimes the propor- 

 tion of hematite becomes so great that the ferruginous bands consist 

 almost entirely of this mineral. They lose then their red colour and 

 appear black or metallic and the rock becomes an iron ore. 



A remarkable microscopic structure is exhibited by one of these 

 rocks (spec. ^Vf). It was collected on the 

 Ur haps S organic! PCr ~ banks of the Son at Agori Khas, the jasper being 

 intercalated with limestone. The hematite 

 particles are disposed throughout the chalcedonic groundmass in the 

 shape of minute hollow spheres or ellipsoids. These are not of the 

 nature of spherulites, for the hematite grains that constitute them are 

 not in contact with one another and are exceedingly small compared 

 with the dimensions of these hollow spheres. In those parts of the 

 slide where the structure is best exhibited, there are two concentric 

 envelopes of hematite dust, and between them is a transparent ring 

 showing the presence of a hollow sphere of silica. These little 

 bodies are distributed all over the slide quite irrespective of the pro- 

 portion of hematite dust. In those portions of the slide where the 

 proportion of hematite is small they become less clearly delineated, 

 while they disappear naturally when this mineral becomes so abund- 

 ant as to render the section opaque. It is where the hematite is 

 abundant, without however interfering with the transparency of the 

 slide, that the structure is best visible and the two concentric 

 envelopes most clearly exhibited. It is possible that the structure may 

 have been originally organic. 



Othqr rocks which also present a jaspidious and banded appear- 



Rocks near «Belawa ance in the hand-specimen contain magnetite 

 peak." instead of hematite. Such is specimen 7J \ 



F 2 67 



