394 L, V. Pirsson—Phonolitic Rooks From Montana. 
Art. XLIL—On some Phonolitic Rocks from Montana ; 
by L. V. Pirsson. 
Or the two rock types described in the following note the 
first was collected from a drift bowlder on the Missouri River 
near Fort Claggett by Mr. W. H. Weed of the U. 8. Geolog- 
ical Survey, the second also from a drift bowlder in the ballast 
material of the Great Northern Railway, at Havre by the author. 
The occurrence of these two closely related types, the one on the 
north and the other on the south of the Bear Paw Mountain, cer- 
tainly renders it probable that they have been derived from 
this little known mountain group and greatly heightens our 
interest concerning it. 
While in the present stage of petrological knowledge the 
description of rocks devoid of their geological mode of occur- 
rence and genetic relationships is of little use or interest, in 
the prétent case the types are so unusual and present so many 
featuresstf petrological and mineralogical interest that it has 
been thought best to give a brief account of them. More- 
over, the rocks have so str ongly marked a habit that it has been 
thought well to call attention to them, since when their place 
of occurrence becomes known, they cannot fail to be of great 
service in tracing the movement of the drift in the region in 
which they are found. They add one more to the hitherto. 
small number of occurrences of phonolite in this country. 
Pseudo-leucite sodalite tinguaite. 
Megascopically, the rock is of a very dark stone color with 
a strong greenish tinge. It is aphanitic, tough, dense and com- 
act. On weathered surfaces it is a clear, rather dark green. 
Through this material, which forms the tinguaitic ground-mass, 
are scattered large white crystals which attain a diameter of 
14°", are sometimes not above a quarter of this in size but 
usually average about 1°" in diameter, and are so thickly seat- 
tered that commonly they are scarcely separated from each 
other by more than their own diameters. They are white to 
pale gray in color and have a greasy, waxy luster. In outline 
they are nearly spherical but generally show more or less erys- 
tal boundaries and most commonly approach octagons, in some 
cases showing the figure quite perfectly. They present in 
fact the different sections which may be obtained by passing 
planes through the icositetrahedron. They appear precisely 
like leucites but are in fact pseudomorphs after that mineral, as 
will be shown later. 
