and other Rooks from Montana. 291 



the grains are very clear, but the larger part are somewhat 

 clouded, not, however, by decomposition and the formation of 

 muscovite and kaolin, but principally by irregular and elonga- 

 ted gas inclusions together with others of indeterminable char- 

 acter. Small crystals of apatite, as well as green and brown 

 hornblende microlites, are noticed in the feldspars. A triclinic 

 feldspar with very fine twinning lamella in only one direction 

 occurs in intimate, almost microperthitic intergrowth with 

 many of the orthoclase grains. This triclinic feldspar which 

 usually is more pellucid than the orthoclase has every appear- 

 ance of being albite. 



Many of the feldspar grains are corroded in a peculiar man- 

 ner as shown on the figures and are filled with faintly doubly 

 refracting analcite. The same corrosion has been observed in 

 an augite-syenite from the Little Belt Mountains,* although 

 the enclosed mineral was not then recognized as analcite ; a 

 comparison with the Square Butte rock shows beyond doubt 

 the identity of the two minerals. It has also been noted by J. 

 Francis Williams in similar rocks.f This analcite is doubtless 

 derived from the albite. It is a curious fact that although there 

 is abundant sodalite present in the rock it should have remained 

 fresh and undecomposed while the albite was attacked. There 

 are no other zeolites or products of decomposition and it ap- 

 pears as if this conversion into analcite would have taken place 

 very soon after the solidification of the rock. It certainly can- 

 not here be regarded as a product of ordinary decomposition. 

 Prof. Brogger^: and other authors have shown that analcite, in 

 general, is the earliest of all the zeolites and that it must have 

 been formed at a relatively high temperature, that is, perhaps 

 between 200° and 400°. Friedel and Sarasin§ for instance 

 obtained analcite by heating the constituents of albite with 

 water to 400°. 



In order to test the character of the feldspars, fragments 

 were introduced into a Thoulet solution. The feldspars began 

 to sink at sp. gr. 2*57 and continued to fall until a sp. gr. of 

 245 was attained. Two portions were subjected to partial 

 analysis : 



Sp. gr. 2 # 5 7 



Sp. gr. 2'55 



Na 2 6-08 

 K.0 8-91 



3-88 

 11*03 



It is evident that the first portion is orthoclase with a strong 

 admixture of albite and equally so that the second is a nearly 



* Eruptive Rocks from Montana, loc. cit., p. 46. 

 f Ark. Geol. Survey, 1890, vol. ii, p. 79. 

 \ Zeitschrift f. Kryst. u. Miner., xvi, p. 169. 

 § Compt. rend. 1883, xcvii, 290. 



Am. Jour. Sci.— TamD Series, Vol. XLV, No. 268.— April, 1893. 

 21 



