180 



ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1917. 



a mineral occurring in small, golden yellow octahedra in this same 

 meteorite, and judged from partial analysis to be an oxy sulphide of 

 calcium and titanium. Free silica in meteorites is extremely rare. 

 Maskelyne described what he considered a rhombic form of quartz 

 as constituting nearly one-third of the siliceous portion of the Brei- 

 tenbach pallasite. The association of free silica in such proportions 

 with olivine and native iron is quite unusual. 



From what is known regarding terrestrial basic igneous rocks, the 

 feldspars of meteorites would naturally be assumed to belong to the 

 more basic varieties, as labradorite and anorthite. Few actual and 

 complete analyses are available owing to the difficulty of securing a 

 sufficient quantity of material in a fair degree of- purity. Those 

 given below from the meteorites of Hvittis, Hessle, and Shergotty 

 show that in at least two instances the feldspar is approximately^ oligo- 

 clase, a form characteristic of rocks of intermediate acidity, as the 

 diorites. The third analysis represents a completely isotropic, color- 

 less mineral forming, together with augite, the essential constituents 

 of the meteorite of Shergotty, and which is regarded by Tschermak, 

 who described it in 1872, as a re-fused feldspar, near labradorite in 

 composition. To this he gave the name " maskelynite." It should be 

 stated that Groth was inclined to regard it as an independent species 

 and allied to leucite. 



The feldspars, it may be said as a general statement, are not promi- 

 nent constituents of meteorites and are limited mainly to those of a 

 basaltic type. In these they occur in the characteristic, lath-shaped 

 forms, polysynthetically twinned. In the chondritic types they oc- 

 cur in the form of sj^oradic gi'anules, sometimes showing twin striae, 

 and in the nearly isotropic maskelynite forms occupying the inter- 

 spaces of other silicates. Concerning the other silicates present, it 

 may be said that the olivines, excepting in the barred chrondritic 

 forms, apparently differ in no essential particulars from those of ter- 

 restrial rocks. The pyroxenes, however, show interesting pecu- 

 liarities. We find, as among terrestrial rocks, both orthorhombic 

 and monoclinic forms, but the first named are the more common. 

 These occur in colorless to grayish — rarely greenish — forms, and in 



