152 



Mr. TS T . Story-Maskelyne on the 



[Jan. 13, 



Viewed as a magnesium calcium silicate, the percentage composition 

 becomes — 



Silicic acid 56' 165 56*604 



Magnesia 23*612 23-585 



Lime 20-223 19-811 



100-000 100-000 



The second column gives the percentage composition according with 

 the formula 



(I Mg, | Ca) O, Si0 2 . 



Such a formula does not accord with those of the ordinary varieties of 

 augite, in which calcium is usually present in at least as high a ratio in 

 equivalents as the magnesium. A deduction, however, of a certain amount 

 of purely magnesian enstatite corresponding in chemical type to the augite 

 has to be made by reason of the presence of the white mineral intercalated 

 in layers along a direction parallel to the plane 0 0 1, and sometimes to a 

 second plane. This white mineral is, there can be no doubt, the mineral 

 next to be described, and its presence would modify the apparent formula 

 of the augite as derived from analysis, increasing the magnesia. 



The trace of the titanoid element in this mineral is included with the iron 

 oxide in the above analyses. 



VII. On the Occurrence of Enstatite in the JBusti Aerolite. 



Besides the augite already described there occurs in this meteorite an- 

 other silicate which constitutes its most important ingredient. The augite 

 is chiefly found in the nodule with the calcium sulphide, and is found more 

 sparsely in the remaining parts. Associated with it throughout, and other- 

 wise forming the chief mass of the stone, is a mineral which, in microscopic 

 sections, presents the appearance of a number of more or less fissured crys- 

 tals of varying transparency, some clear, some nearly opaque, and usually 

 presenting a not very unsymmetrical polygonal outline. Those crystals 

 are imbedded in a magma of fine-grained silicate, itself often entangled in 

 an irregular meshwork of opaque white mineral. Amongst these ingre- 

 dients, when mechauically separated, what seems to be three different 

 minerals can be distinguished. The rarest of them is transparent and 

 colourless, and very irregular in the form of its fragments ; a second is of a 

 greyish-white colour, translucent, and offering an even less hopeful pro- 

 blem to the crystallographer than that presented by the first. The third 

 is an opaque mineral with a. distinct cleavage following the faces of a prism 

 of about jjY^p au ^ w ^ tn a secou d imperfect cleavage perpendicular to the 

 former. From a few fragments of the two former kinds some measure- 

 ments were obtained, which conduct to the conclusion that, like the last- 

 mentioned silicate, these minerals are enstatite. The angles 1 0 0, 1 1 0 are 

 46° 25', and 10 0, 1 0 1, 41° 34'. 



Chemical analysis confirmed the identity of these three minerals by 



