i 



294 Journal of the Astatic Society of Bengal. [N.S., XIII, 



9 



in detail, and I think that a record of the results of my study of 

 the Hamandun limburgites will net be out of place. 



„ . colour, with 



a specific gravity of 294. ' The rock weathers superficially 

 into a whitish lithomargic substance, while the removal of the 

 lithomarge very often gives rise to a pitted appearance on the 

 surface, the unweathered black part often standing out in relief. 

 The lithomarge sometimes shows a reddish tinge. The por- 

 phyritic structure of the rock is very well seen even in hand 

 specimens, and a few good crystals of augite were isolated. One 

 of them was carefully examined and the faces e, m", a and m 

 were determined. One isolated small crystal, when treated with 



hydrogen peroxide, turned slightly yellow, showing the presence 

 of titanium. 



Thin sections of the rock, when examined under the micro- 



scope, show its porphyritic structure. The phenocrvsts found 

 are titaniferous augite and olivine, and both of them have been 

 changed to serpentine. The augite is pale- violet in colour with 

 a very faint pleochroism, often twinned and showing a zonal 

 structure. Serpentinisation is fairly marked, though, as one 

 may expect, it is not so completers in the case of olivine. 

 Some iron ores are also present, evidently due to the breaking 

 up of the ferro-magnesian silicates. The groundmass is hypo- 

 crystalline* consisting of microlites of lath-shaped felspars and 

 of augite. in second generation, with an amount of residual 

 glass. The glassy portion of the groundmass is of brown colour 

 with yellow spots and patches due to the formation of serpen- 

 tine. This serpentine is possibly derived from the alteration of 

 some of the augite microlites in the groundmass, and it is ex- 

 tremely difficult to decide whether olivine was present in the 

 groundmass or not— the whole of the olivine having in the former 

 case been replaced by serpentine. 



A study of thin sections shows two peculiarities of the rock 

 viz.. (i) the smaller amount of the phenocrysts of olivine while 

 compared with those of augite and (ii) the presence of felspar 

 microlites in the groundmass. As regards the relative propor- 

 tion between olivine and augite, phenocrvsts, it mav be men- 



Dr 



peculiarity later on. 



awn 



iw \t ch emical examination of this rock was kindly undertaken 

 pLf,' £™ hodh ? Chan <*ra Chatterjee, M.A.. F.C.S., of the 

 bengal Chemical and Pharmaceutical Works. This analysis 

 snows conclusively the ultra-basic nature of the Hamandun 



Sen.B^c 9 ™ *" avera ge of four determinations by Babu Bijoy Gopal 



» JGS vIi°Y? y ,T f0r Stude "t« (4th edition), p. 208. 

 ^.J.US., Vol. XLV. pp. 352.5 (1889). 



