3ti MIDDLEMISS: THE GEOLOGI OF IDAIi STATE. 



in its general appearance and in its composition. It is rather fine- 

 grained, grey in colour and sparsely speckled with dark green. It 

 penetrates the calc-gneiss in veins usually parallel to the foliation 

 of the latter, both veins and foliated gneiss being obliquely trun- 

 cated by the Idar granite of the centre of the hill. Microcline is 

 the dominant felspar and shows very generally Carlsbad twins 

 superposed on the ordinary microcline twinning. There is a little 

 plagioclase, the albite twinning lamella? extinguishing at angles 

 of 11° to 13°, and being therefore probably albite-oligoclase. Quartz 

 is in fair abundance. The dark minerals are green-yellow horn- 

 blende, a little iron ore and zircon. 



Specimen no. 5 - ;> g (12318), from lj mile west of Babsar, also 

 a vein in the calc-gneiss, is again very characteristic of these grey, 

 mottled, medium-grained granite-aplites. The thin section shows 

 microcline with Carlsbad twins predominant, quartz, and a great 

 deal of what appears to have been bleached biotite in nests, 

 altered now to a crypto-crystallinc aggregate of chlorite. There 

 is a very little iron ore, and garnet appears here and there in the 

 hand-specimen, but there is none in the slide. 



Specimen ffi 6 (12319, PI. 10, fig. 3), occurring in veins in the calc- 

 gneiss on the west side of Dharol hill, is another rock generally similar 

 to 5 - -q just described, the dark minerals being almost entirely 

 garnet, and even this is very sparingly developed. There is abund- 

 ance of quartz and the felspar, much kaolinised, appears to have 

 been chiefly microcline and plagioclase. There is no iron ore. 

 Specimens £fe and 4 9 9 ' 8 (12320), also from the west side of Dharol 

 hill, are similar examples with biotite arranged in nests and streaky 

 lines, the quartz-felspar being much as in ^ f f 6 . The biotite is 

 very scanty and is in ragged plates and irregular aggregates of 

 plates with pale yellow and dark brown pleochroism, intimately 

 intergrown parallel with the uralitic hornblende. There is a very 

 little iron ore associated with the biotite, and some apatite. 



The last three examples from Dharol hill constitute an in- 

 structive series as regards the ferro-magnesian minerals, which 

 always appearing in blurred and indeterminate patches in the 

 hand-specimens, are observed under the mieroscopc to be composed 

 of garnet, biotite, and biotite intergTown with uralitic hornblende 

 in varying mixed proportions. 



Specimen ^\ (12321), from the boss on the ridge 2 miles N.N.E. 

 of Vadali, and 4 \- 2 (12322), from the south end of Dharol hill, both 



