178 THE ROMAN COMAGMATIC REGION. 
few flows, the chemical and mineralogical features are very similar, though there 
are some marked differences. 
If the magmatic positions, which express concisely the general chemical char- 
acters, of the various analyzed rocks of the Central Montana Region be examined 
and compared with that on p.n, the main points are clearly seen. 
In the first place, there is a considerably greater variety of magmas than in the 
Roman region, 27 being represented in central Montana against 17 known in central 
Italy. But the general range is much the same. There are rocks belonging to 
the persalane, dosalane, and salfemane classes, but none in the dofemane (except 
one on the border of salfemane) or perfemane classes. While there are some rocks 
which belong to the quaric orders, britannare and austrare, in the persalane and 
dosalane classes, the great majority are either perfelic or are more or less lenic, and 
it is noteworthy that in both regions rocks containing notable amounts of norma- 
tive lenads are rare in the persalane class, become more frequent in the dosalane, 
and are most so in the salfemane. Peralkalic rangs are likewise unusual, though 
more common in the Montana Region than in the Roman, and in the former they 
are met with in rocks of the dosalane and salfemane classes, which is not the case 
in the latter. Alkalicalcic rangs are rare in both, and in both also the rangs most 
commonly met with are domalkalic. In the subrangs the majority of those of the 
Montana Region are sodipotassic, but with a very considerable representation of 
dopotassic ones, while we have seen the reverse to be true of the Roman Region, 
and in addition a few dosodic subrangs are found in Montana sporadically, which 
are entirely absent in the Italian one.* But on the whole potash can be said to 
dominate over soda in both, though this is less strongly marked in the Montana 
rocks. The resemblance between the two regions is also shown among the minor 
constituents by the prevalence of high BaO in the rocks of both, a point which 
will be referred to later. 
Similarly, so far as the serial chemical characters are concerned, we find in 
both a great agreement in that potash increases relatively to soda as silica decreases 
and as the femic components increase,! and also that magnesia increases over 
ferrous oxide in the same direction. It will also be noted that in both the general 
geographical distribution of magmas is much the same. The border or end dis- 
tricts are more highly siliceous and salic and with more soda relatively to potash 
than the central districts, which are much more distinctly femic and lenic and more 
markedly dopotassic. 
The average magma of the Central Montana Region has not been calculated 
by Pirsson, but he givesj an estimate of that of the Highwood District, a comparison 
of which with the averages of the magma of the Roman Region and of the Cam- 
panian District is not without interest. 
* It is worthy of note that dosodic subrangs are very common in the Crazy Mountains, which Pirsson does 
not include in his region. The rocks of these evidently belong to a different and more highly sodic magma. 
t Pirsson, op. cit., p. 43. 
$ Pirsson, Bull. U. S. Geol. Surv. No. 237, 1005, p. 193. 
