40 MUXDLEMISS : THE GEOLOGY OF IDAR STATE. 



Allanite Aplite. 



Two miles N. by E. of Khed Brahma, and forming a long 

 strangling tongue of scarcely exposed rock, 



With Allanite. rurm in«r out into the plain southward from 



the main hill-mass behind, is an exceptional variety of the vein 

 granite permeating the calc-gneiss, £ft, (12330). It is remarkable 

 for the fact that the only dark minerals represented m it are allanite 

 and sphene. The allanite largely predominates in rather small 

 elongated grains £ to \ inch long by one-sixteenth inch wide. 

 The grains are roughly aligned more or less parallel to the 

 edge of the vein, making a rough streakiness not amounting to 

 banding. They are sometimes of good crystal outline, sometimes 

 ill-defined grains constituting a centre for some other decomposed 

 mineral (1 diopside or uralite). The allanite is of pitchy lustre 

 and fracture, black colour (greenish by transmitted light, when 

 it has the appearance of bottle glass). Under the microscope it is 

 of pale bluish-green colour, centrally, with an outer yellowish- 

 brown or orange-coloured border, which sometimes extends into 

 the other minerals of the rock, staining them. It is not pleochro.c, 

 bein" generally also abnormally isotropic, except for a few patches 

 which show double-refraction tints. It has no regular cleavage, 

 but a set of more or less parallel cracks traverse the crystal. 

 Radiating cracks also traverse the mineral surrounding the allanite 

 in neighbouring specimens (£&, 12331 , H. 1 1 , fig. 1 and £&, 12332). 

 Before the blowpipe it swells up slightly and glows a little, yielding a 

 greyish slag-like mass. It dissolves in hydrochloric acid, but not after 

 heating. 1° hesitated to choose between allanite and gadolinite for 

 the mineral until a chemical examination of it, made for me by Mr. 

 Tipper in the Geological Survey laboratory, gave the following 

 results :— specific gravity 3-12 (which is much too low for gadolinite) ; 

 it contains much water, and gelatinises at once with strong hydro- 

 chloric acid; it contains silica, iron, alumina, cerium earths with 

 traces of yttrium and erbium, calcium and the alkalies, but no 

 beryllium. From this analysis the mineral is clearly not gadolinite. 

 It is associated in the rock with a fair amount of sphene, some 

 zircon besides much microeline, plagioclase with bent lamellae, 

 and quartz. In specimen *& (12332), a variety found near /& 

 there is also present uralitic hornblende m ragged plates and long 

 section. Other specimens are ^ and &% numbered m order 

 according to their proximity to *f . The bands of the granite 



