Vol. 56.] THE GEOLOGY. OF GILG1T. 341 



like that now described, however, is difficult of exact determina- 

 tion, as the mineral remains more or less dark during a revolution 

 of 20°, and it is hard to say when the maximum is reached. 

 Moreover, the maximum extinction with the vertical axis seems to 

 vary in different species. It is usually said to be from 34° to 37° ; 

 but a specimen, quoted by Dana from Hittero, 1 showed it at 

 40° or 41°. 



The structure of the Baltit Hornblende-Granite is typically holo- 

 crystalline. It bears marks of strain, but tessellated quartz is 

 extremely rare. 



(2) The Hatu Pir Granite. 



Hatu Pir, 2 a mountain rising 10,254 feet above sea-level, situated 

 near the connueuce of the Astor and the Indus, is composed of 

 this granite. It is a rock which always contains dark mica 

 as an original constituent. The mica is strongly dichroic, and 

 usually polarizes brilliantly ; occasionally it is altered to chlorite. 



The quartz in one slide equals the felspar in amount, but in the 

 others it is less abundant than the felspar. As a rule, orthoclase 

 predominates over plagioclase: the latter is mainly oligoclase, 

 with an occasional mixture of andesine. The oligoclase was one 

 of the first minerals to crystallize out of the magma, and crystals of 

 it are enclosed in the orthoclase. Zonal structure and twinning on 

 the Carlsbad, pericline, and albite laws are common. The felspar 

 is sometimes fairly idiomorphic. 



Muscovite, or silvery mica, occurs in a few slides, but it 

 may be a secondary product. A little magnetite is almost always 

 present ; epidote, sphene, and allanite also occur. 



More or less apatite is found in every slide save one. It is 

 granular in form, and rarely exhibits crystallographic outlines. 

 This habit is very characteristic of the apatites in the Gilgit 

 granites. 3 The hand-specimens react strongly for phosphoric acid ; 

 but in order to remove all doubt about this mineral, I subjected a 

 thin slice, after examination under the microscope, to the action of 

 nitric acid. This completely dissolved the colourless granular 

 mineral, and the solution thus obtained reacted unmistakably for 

 phosphoric acid, showing that it was apatite and not zoisite. 

 Optical tests were also applied. 



Zircons are present, but are only occasionally seen in thin 

 slices. Schorl is still more rare. 



Some of the hand-specimens of this granite show decided parallel- 

 ism of structure, but this is not observable under the microscope. 

 Marks of strain, however, are usually present in some form or 

 other ; strain-shadows are common ; felspars are cracked ; and 

 portions of the quartz break up under crossed nicols into a 



1 ' System of Mineralogy' 6th ed. (1892) p. 523. 



2 Named after a Mahomedan saint. 



3 Zirkel remarks on ' the apatites in massive granites being generally far 

 more broad and short' than the long thin prisms in certain other granites 

 described by him, ' U.S. Geol. Explor. of 40th Parallel ' vol. vi (1876) p. 49. 



