Weed and Pirssun—Igneous Rocks of Montana. 477 
Syenite. Yogoite. Shonkinite. 
eee, abe Raa, 56 80 1°44 
Belek A Seep Ate ame 27 32 43 
HERO pba Es, 22 | he ae 59 98 
The change is of precisely the same character as with the 
main oxides, there is a progressive increase toward the basic 
end, the titanium is concentrated in the iron ore and the phos- 
phoric anhydride shows itself in the larger amount of apatite 
present, while the function of the barium is somewhat uncer- 
tain. Instances of intrusive stocks which vary in composition 
in different parts of the same mass are well known, such as 
for instance that of Carrock Fell, which has been so ably 
described by Harker,* where the mass grows steadily more 
basic toward the margin, or that of Ramnis mentioned by 
Brégger,+ where the reverse is the case. In his recent import- 
ant monographt Brégger names a rock series which occurs in 
one individual mass, and which has been formed by the dif- 
ferentiation of the mass in its final resting place (“ laccolitic 
differentiation ”’§), a ‘“ Hactes suit” to distinguish it from a 
series of rocks formed from independent magmatic eruptions 
which show also a continual progression or gradation in their 
mineralogical and chemical composition. The series is termed 
a “ Rock Series” (Gesteins serie) in the latter case. The Yogo 
Peak mass shows an excellent example of the first class, but it 
is also to be expected that in many given regions a “facies 
suit” in one erupted mass will correspond to a “rock series” 
which may be developed in the region at large. 
This may be shown, in part at least, for the Yogo district by 
taking in addition to the series of analyses of Yogo Peak three 
others, one of a quartz syenite porphyry constituting an 
immense uncovered laccolite at Big Baldy Mt., some miles 
northwest of Yogo, and one of a rather coarse-grained augite 
minette that forms a thick intrusive sheet a number of miles 
southwest of Yogo, one of a great series that is genetically 
connected with it in the Little Belt Mts. series of intrusions. 
These are taken from very complete analyses by Dr. Hille- 
brand, but here only the important rock-making oxides are 
considered. To these is added also the analysis of the shon- 
kinite from Square Butte previously given. 
* Quar. Jour. Geol. Soc, vol. 1, p. 311, 1894, vol. li, p. 125, 1895. 
+ Zeitschr. f. Kryst, vol. xvi, page 45, 1889. 
+ Gesteine der Grorudit-Tinguait Serie, p. 179, 1894. 
§ Loe. cit, p. 153. 
