78 HATCH: THE KOLAR GOLD-FIELD. 



handsome specimen (No. J 4 from the Ooregum Mine) photographed for the 



frontispiece, contains an abundance of calcite filling in small " pockets " produced 

 by the folding of the schist, which shows the usual coarse-grained pyroxenic bands 

 and brown-mica films near the auriferous lode. 



Under the microscope, the hornblende schists are seen to be composed very 

 largely, almost wholly in many cases, of green hornblende, which characteristic- 

 ally forms distorted, sheaf-like crystals presenting the following pleochroism : — 



»= straw-yellow. 

 b= dark-green. 

 e— blue-green. 

 Extinction ( c : t )=i6°. Twins parallel to (ioo) not uncommon. 



Brown mica, almost uniaxial, is commquly associated with the hornblende in a 

 way which suggests its secondary development from the latter. Occasionally in 

 radiating bundles of hornblende some of the radia consist for a part of their length 

 of brown mica and the association is one which from previous experience might be 

 expected as an alteration product of hornblende in rocks which have been subjected 

 to the action of deep-seated vapours. 1 The sections of the hornblende schists are 

 generally spotted with numerous granules of opaque iron-ores, more frequently 

 pyritous than black. Rounded granules of sphene are common in most sections 

 and sometimes in large grains show a distinct pleochroism. The colourless 

 minerals are generally very irregular in their distribution and seldom abundant 

 over a large area. Granular quartz, plagioclase-felspar and a white mica, formed 

 by secondary alteration of the felspar, are the usual constituents of the colourless 

 patches. 



A noteworthy feature frequently seen in specimens of these hornblende schists 

 is the occurrence of bands, half an inch wide or less, of pale-green augite, which 

 shows the characters of malacolite and occurs in irregularly-shaped crystals, often 

 intergrown in a pegmatoidal fashion with calcite. These bands show no signs of 

 crushing and do not strictly conform with the foliation planes, but cut them 

 obliquely as is also the case with the associated quartz veins, and they not impro- 

 bably originated under similar circumstances. The malacolite has in its turn 

 undergone a partial change into actinolitic hornblende, and field observations may 

 possibly show that the bands are old enough to have suffered from the folding 

 movements which are known to have disturbed the associated auriferous quartz 

 lodes. Fragments of the same kind of pyroxene are frequently found isolated in 

 the quartz veins, the ragged, spongy crystals of malacolite sometimes enveloping 

 numerous granules of clear quartz. 



Zoisiie is a frequent constituent of the hornblende schists ; but is apparently 

 quite irregular in its distribution. 



Only a few of the specimens from Kolar show unmistakable diabasic structure' 

 the felspars retaining their ordinary elongated character with ragged outline, and 

 sometimes with the hornblende around showing an ophitic disposition. Such rocks 

 one might safely refer to epidiorites, and there are one or two others, now in a 



1 C/. — Parsons on " The development of brown mica." Geo 1 .. Mag., July 1900, pp. 316-319 

 and literature therein quoted ; also Van Hise, " Principles of North American Pre-Cambrian 

 Geology, " p. 690. 



