262 J. ParMnso)i — TJie Making of Quartz Schist. 



noticeable degree. Hence the strain produced by tbe pressure the 

 rock has undergone has been removed from the greater part of 

 the rock, while the larger and obviously clastic grains still bear 

 its marks. 



The rock does not appear to have undergone serious deformation 

 since it assumed its present condition, and as regards the state of 

 the quartzes at the time of maximum pressure we are met with 

 evidence of two and diverse kinds. Firstly, we find clear indications 

 of incipient cracks, conical in section but with slightly concave 

 outlines, which are usually characteristic of mineral matter stressed 

 beyond its elastic limit, and pointing to the crushing of a practically 

 rigid solid ; whereas, secondly, the irregular shape of the quartzes 

 under the microscope are suggestive of a plastic deformation, e.g. 

 some have been apparently bulged out by an intruding finger of 

 the rock, or the like. 



On the whole I conclude that the constituents of the rock have 

 been partially plastic under the stress to which they have been 

 subjected, but at the same time sufficiently rigid to have cracked 

 in the process. 



With the assumption of these characters disappears the smashed - 

 looking aggregate so universally indicative of crush. The mica 

 clearly has been formed in situ, and the production of the quartz 

 mosaic by crushing and subsequent reconstitution could not be 

 safely pi'edicated simply from an examination of this part of the 

 section. The gi'ains of felspar scattered throughout the slide are 

 plentifully sprinkled by minute crystals of mica, or slightly dis- 

 coloured by brownish dust, but otherwise are quite translucent, 

 and show well-defined polysynthetic twinning (12° to 15°). We 

 find, then, almost a new rock ; the traces of original quartz grains 

 are few in number ; the mica has been formed in situ and does not 

 suggest a clastic origin ; and the greater proportion of the felspars 

 have undergone reconstruction. 



2. The Quartzites of Delhi and Jaipur. 



Of great antiquity ^ and greatly altered, the old quartzites of India 

 near Delhi and Jaipur yet show no resemblance to the rocks above 

 described. A specimen taken from the celebrated 'Ridge' at Delhi 

 consists of quartz and thickl}' scattered, exceedingly small crystals 

 of various minerals. The most common of tliese are shapeless flakes 

 and scales of a quite colourless mica, averaging roughly -OOo inch 

 across, distributed without any orientation through the rock, 

 commonly at the junction of two or more quartzes. Indications of 

 the basal cleavage are usually wanting, and there is no evidence that 

 the mineral is detrital. In addition, we find flakes of a gummy 

 brown to greenish brown tourmaline (and possibly some of brown 

 mica), many of which are exceedingly minute, often not more than 

 ■0006 inch in diameter, and which usually exhibit rounded outlines, 

 especially in the smaller grains. Other constituents occur, but are 



^ They are a member of the transition system, i.e. they occupy an intermediate 

 position between the gneisses and the older Palaeozoic rocks. 



