358 Weed and Pirsson — Bearpaw Mountains, Montana. 



withdraw the term " yogoite " and substitute " monzonite " in 

 its place and thus avoid confusion in the nomenclature. 



In connection with the associated occurrence of this monzo- 

 nite and the alkali syenite previously described, it is of interest 

 to recall the keen remark of Rosenbusch* on the probable pres- 

 ence of an alkaline syenite at Monzoni. 



Leaving aside then for the present Brogger's radical proposal 

 for the new group, the augite syenite family, taking into con- 

 sideration the relative quantities of light and dark minerals 

 according to our method of classification f would be as follows: 

 sanidinite, augite syenite (akerite and laurvikite types), monzo- 

 nite, shonkinite, pyroxenite. This leaves out of consideration 

 the plagioclase in the monzonite ; if it is considered and Brog- 

 ger's monzonite group be adopted, a gap is left to be filled by 

 some type of hitherto undescribed alkaline syenite, rich in 

 augite and free from plagioclase. 



Gradational types between the monzonite and the more basic 

 shonkinite were collected and their occurrence is quite what we 

 should expect. 



Shonkinite type. — The shonkinite type, the complementary 

 rock of the acidic syenite, occurs in the outer or peripheral 

 part of the stock. This rock crumbles readily in weathering, 

 and good exposures do not occur. Pyritic impregnations have, 

 however, led mining prospectors to drift into the rock, and the 

 tunnel face and dump heaps afford quite fresh unaltered mate- 

 rial. At the Zortman claim the rock is mottled and streaked 

 with feldspatfyic material often in stringers and slender extended 

 portions (Schlieren), and large irregular feldspar crystals occur. 

 The normal rock is, however, free from these streakings, the 

 specimen analyzed being entirely devoid of such material. 



It is a very dark, basic-looking rock of a moderately coarse 

 grain and strongly micaceous appearance from the light reflected 

 from innumerable cleavages of biotite. In the description 

 / which follows, the type from the Bearpaw mine referred to in 

 the first part of this paper is included, as it differs in no essen- 

 tial from the present occurrence. 



Under the microscope it is found to consist essentially of the 

 same minerals as the monzonite previously described, though 

 in different proportions ; they are apatite, iron ore, diopside, 

 biotite, alkali feldspar, and in addition a very little olivine and 

 probably a little nephelite. 



The apatite is present in short, stout crystals which are clear 

 and colorless. The diopside is present in rather long, slender 

 prisms and in short, round anhedral grains ; it is of a very clear 



*Mikr. Phys. Mass. Gest, 3d ed., p. 124, 1895. 



fSee this Jour., Rocks of Togo Peak, p. 478, vol. 1, 1895. 



