322 Weed and Pirsson — Missourite. a new Leucite 



alumina. This is unfortunately an all too common error in 

 rock analyses. One of the best is shown in the above table in 

 No. lY, and it will be seen that the agreement is good in the 

 essential details. In No. II is given one of the absarokites of 

 Iddings, with which the missourite, from a chemical point of 

 view, seems to be closely related. In No. Ill is shown the 

 composition of the shonkinite from the same mountain group. 

 With the same amount of silica in each, the lower alkalies of 

 the shonkinite have permitted orthoclase to form as the domi- 

 nant white mineral, while their higher amount in the mis- 

 sourite has produced leucite in its place. In the shonkinite the 

 excess of the alumina over the alkalies has gone into the augite 

 and biotite, and the same is undoubtedly true in the missourite- 

 Taking into consideration the ratios shown by the analysis, the 

 separations by the heavy liquid and the study of the section, 

 the rock has approximately the following mineralogical compo- 

 sition : 



Iron ore 5 



Augite ._ 50 



Olivine 15 



Biotite 6 



Leucite 16 



Analcite 4 



Zeolites.-- 4 



100 



Structure. — The structure is purely granitoid, but is not 

 hypidiomorphic since no mineral shows any crystal planes, but 

 all are wholly allotriomorphic. The iron ore, apatite and 

 olivine commenced forming before the other minerals, but are 

 in lounded anhedral grains ; the augite and leucite were crystal- 

 lizing contemporaneously, as shown by the fact that each 

 encloses grains of the other. In plain light the rock section 

 appears precisely like those of many coarse-grained, massive 

 gabbros, and it is not until the nicols are crossed that it is per- 

 ceived that the colorless areas are not composed of striated 

 plagioclase but of isotropic leucite. 



The relations of the minerals are shown in the accompanying 

 figure. 



Classification. — It is clear from what has been said in the 

 foregoing that this rock is a new type, and it fills a place which 

 has hitherto been vacant in all systems of rock classification in 

 which either the texture, structure and granularity of rocks or 

 their geological mode of occurrence is taken into account. It 

 is the massive, granular, plutonic representative of the leucite 

 basalts and bears the same relation to them that gabbro bears to- 



