ANALYSIS OP SQUARE BUTTE PYROXENE. 411 



In the foregoing analysis the rock was crushed, sifted, and the result- 

 ing powder washed and then separated hy the use of Retgers' * silver- 

 thallium-nitrate fluid in the ai)paratus devised by Professor Penfield,t 

 and by this means, aided by tlic magnet, material of exceptional purity 

 was obtained. 



The comparison of the ratios in the analysis shows that CaO to (Fe]Mg)0 

 is as 1 to 1, and that the dio})side molecule is thus chiefly i)resent. The 

 presence of the alumina suggests that Tschermak's molecule KAL SiO^ 

 must also be present. If we subtract from the sum of the RO molecules 

 enough to make the number of the R,,0 equal to that of the R.^0., and 

 take out the same number of 8iO.^ molecules, the following table shows 

 the composition of the augite : 



R(? = "0386 } ■^^'^^ • ^-^=^ = '^^^^ • ^^^-^ "" -^'^'^^ : : 1 : 1 : 1- 

 RO = .7790 : SiO, =- .7710 : : 1 : 1.01. 



The very striking agreement of these ratios with the theory must cer- 

 tainly be held to add another very strong proof to the correctness of 

 Tschermak's assumed molecule. The augite then has almost exactly 

 the following composition: 18 Ca (MgFe) SiA+'-^ (^X^) (AlFe), SiO^. 



Since the qualitative analysis of the feldsi)ars has shown the absence of 

 lime, if we deduct enough from the amount found by the mass analysis 

 of the rock to turn the i)h()sphoric anhydride into ai)atite, a comparison 

 of the remaining amount, 11 per cent, with the 22 i)er cent of lime de- 

 manded by the pyroxene, shows this mineral forms one-half of the rock 

 by weight, a fact which agrees with the appearance of the hand specimen 

 and the study of thin-sections. 



Orthoclase. — The predominant feldspar is orthoclase. This is shown 

 by the study of thin-sections, l)y the separation of the feldspathic con- 

 stituents by heavy liquids, and may also be inferred from the chemical 

 analysis of the rock where potash is seen to greatly predominate over 

 soda. The mineral is quite fresh and wholly allotriomorphic, its shape 

 being determined by the angular interspaces between the pyroxene in 

 which it is found. Sometimes it assumes rude lath-shaped forms. 



It is apt to be filled with fine interpositions whose exact nature can- 

 not be told. They commonly possess the form of their host and their 

 longer axis coincides with that of the crystal, and, so far as can be deter- 

 mined, they are arranged in i)lanes parallel to prism faces. Tliey do not 

 contain ])ubbles, the reflection ])and surrounding them is narrow and 



*jHhrbuch fur Min. 18'J3, vol. i, p. 9(). This most hiippy discovery of Profestior W. Retgers has 

 pliiced ull working minernlo)?i«ts an«i petrographers deeply in hin debt. 



t We desire to express our thanks to Professor S. L. Penfield for kindly aid in making tiie sepa- 

 ration in apparatus recently devised l»y him for the special use of the Retgers' tiuiil, and Ijy means 

 of which the operation may be carried on wiiii nearly the sumo ease at»d with all the (crtjiinty of 

 the usual heavy liquids. 



