148 Weed and Pirsson — Bearpaio Mountains, Montana. 



SiOs -264 =2 ^ 

 CaO -132 =1 I Diopside 

 MgO-118)_, { Bl-1% 

 FeO 014 j" ~ J 







SiOa 



•511 =4-) 



FesOs 



•047 = 1 [Iron Ore 

 •047 = 1 C 11-8^ 



AI2O3 



•115 =1 ' Leucite 

 •038 ) , f 57-1^ 

 •092 [ -^ ^ J 



FeO 



Na^O 







K2O 



Thus the analysis and the mineral composition agree in the 

 most satisfactory manner. The iron ore is mostly present in 

 the fine globulitic material which forms the main part of the 

 base. 



The presence of the water is difficult of explanation unless 

 we consider a certain proportion of analcite present among the 

 leucites. The rock appears absolutely fresh and unaltered, 

 and the amount of glass base is entirely too small to contain it. 

 Lindgren* and Cross f have shown that analcite may be 

 present in otherwise perfectly fresh unaltered rocks of high 

 alkali content, and it is regarded by them as a primary 

 component. 



Analyses of other leucite rocks are given in the table for 

 comparison, and it will be seen that the Bearpaw rock agrees 

 most closely with the Yesuvian lavas : they are distinguished 

 from it by higher amounts of lime and alumina and less of 

 iron. The only two occurrences of leucitic rocks in this 

 country of which we have analyses are given in E^os. 4 and 5, 

 from which the present typ'e varies considerably. It seems 

 probable from the chlorine given in No. 5 that part of the 

 leucite should really be referred to sodalite, and that 4 per 

 cent, may be present. 



The only other occurrence of leucite in America that we are 

 aware of is the rock from Lower California described by von 

 Chrustschoff,:}: though Kemp has shown that a dike in J^ew 

 Jersey should unquestionably be referred to leucite tephrite.§ 



The occurrence of pseudo-leucite is not here considered.] 



* Proceedings Cal. Acad. Sci., ser. 2, vol. iii, p. 51. 



f Geol. Cripple Creek District, Colorado, p. 36. 16th Ann. Kept. Director U. S. 

 G. S., Part II, p. 36, Washington, 1895. 



X Tscher, Mitt., vol. 6, 1884, p. 160. 



§ This Journal, vol. xlv, 1893, p. 298; ibid., vol. xlvii, 1884, p. 339. 



II As this article was passing through the press, we have received the annual 

 report of Dr. Hoffman (Geol. Surv. Canada, Part R, vol. vii, p. 13, 1896), in 

 which he states that dark gray boulders containing large sub-translucent smoke- 

 gray leucites have been found by Dr. G. M. Dawson, in the gravel of the Horse- 

 fly mine on Horsefly River, Cariboo Dist., Brit. Columbia. It is to be hoped that 

 the Canadian geologists may be able to follow up this interesting occurrence. 



