Weed and Pirsson—Igneous Rocks of Montana. 479 
The present work has shown that, starting with the most 
acid form, we have at Yogo Peak a series of partial magmas 
with gradually rising lime, iron and magnesia, falling silica, 
alumina and soda, with potash as the dominant alkali. These 
erystallized into a series of massive, evenly granular rocks com- 
posed chiefly of augite as the ferro-magnesian mineral, but 
with accessory iron ore, biotite and possibly amphibole and 
olivine, while the feldspathic component is essentially ortho- 
clase with which may be associated accessory plagioclase. For 
this series (adding the natural extremes not found at Yogo) we 
propose the following classification : 
All orthoclase, no augite = Sanidinite. 
Orthoclase exceeds augite = Augite-syenite. 
Orthoclase equals augite = Yogoite. 
Augite exceeds orthoclase = Shonkinite 
All augite, no orthoclase = Pyroxene and peridotite rocks 
of various types. 
Such a method of classification is a natural one and it does 
not present hard and fast arbitrary boundaries, but leaves a 
certain amount of elasticity in the determination of the types. 
Within certain lines it must also determine the chemical com- 
position. Under the term augite is, of course, included the 
other accessory ferro-magnesian minerals, as the accessory feld- 
spathic ones are included under the orthoclase. 
Summary and Conclusion. While there are many points 
of interest connected with the Yogo massif, which in the brief 
limits of this article have not been touched upon, such as its 
contact phenomena, the occurrence of aplitic dikes cutting it 
and its connection with radial dikes and encircling intrusive 
sheets of various rocks as well as questions of theoretic petrol- 
ogy to which its discussion must give rise, we have endeavored 
to present the following facts, which will be of general interest. 
That Yogo Peak is composed of a core or stock of massive, 
granular, igneous rock, and that this rock is composed chiefly 
of augite and orthoclase. That the mass shows a progressive 
differentiation along its east and west axis, with a continual 
inerease in the ferro-magnesian elements over the feldspathic 
ones. The resultant rock types have been classified into three 
groups: syenite, where feldspar exceeds augite; yogoite, where 
they are practically equal, and shonkinite, where the augite 
dominates, the latter being similar to a rock type previously 
described. 
Washington and New Haven, October, 1895. 
