Weed and Pirsson—Igneous Rocks of Montana. 473 
Analysis of Yogoite. 
SUS ee 54°42 
CO Se re a MS es = 80 
BN Or Ge ae teed Dae ele o 14:28 
Or, O.ece yy geere. . trace 
LANG ewes epee sas ae 3°32 
12 0 a a eee ee 4°13 
ANNO ye eee eo 10 
1 it OES Sa ae tan ve eee 6°12 
(SE CREE ae nen Gere iret: Ser 772 
Peet ne i eee ee. | MD 
SS ie eee eit ny: toe chetiianes 13 
One es ee eee oA 
CLG Selita aha ha adabeaclersie! ( ssci hnaisbadlage tp 
12 ag ie ae gs trace 
EO rat £108 J enaaes eee dh 29 
EVOr above LTO. 2.2 See he "38 
OST OPO? 25 "59 
100°19 
The features of this analysis are the moderate silica and alu- 
mina percentages, with the high amount of iron, lime, magne- 
sia and alkalies. Although the minerals (if not quantitatively 
expressed) are those of the syenite, its chemical composition 
removes it very far from any typical syenite. The chemical 
characteristics are, indeed, those of the lamprophyre group, 
yet it is a fairly coarse-grained, evenly granular rock, forming 
a transition phase in the center of a great stock. It is to this 
type that we have given the name of “ Yogovte,” and the rea- 
sons for so doing and its systematic position will be discussed 
later. 
Shonkinite. West knob rock type.—The character of the 
outcrops occurring on the western end of the peak differs some- 
what from those which have been previously described. The 
rock does not possess the thick, platy parting that prevails to 
the east, but has an exceedingly massive character, giving rise 
to bold, heavy crags and castle-like forms, often of curious 
shapes, which rise abruptly from the small grassy plots lying 
between them. The rock is exceedingly tough and breaks 
under the hammer with great difficulty. It has changed grad- 
ually in character until here it is as much darker than the 
Yogoite of the middle knob as the latter is with relation to 
the syenite. On a fresh fracture the rock is of a very dark 
stone color, and at first glance recalls many coarse, dark gab- 
bros. On inspection it appears that the quantity of ferro-mag- 
nesian minerals is very large, and the eye is caught by the 
reflection of numerous plates of a dark brownish biotite, which 
average several millimeters in diameter. With the lens a great 
