COMPARISON WITH OTHER REGIONS. 
It is only within the last few years, and following the increase in the number 
and quality of chemical analyses of igneous rocks, that comagmatic regions (petro- 
graphic provinces) have been described in terms that permit of satisfactory corre- 
lation between them, with a view to their bearing on the phenomena and processes 
of differentiation. And even now the number of those which have been at all 
adequately described is so small that it would be unwise to attempt any broad gen- 
eralizations. These must wait for the future and a great increase in our detailed 
knowledge, especially as regards the chemical side, of many widely diverse regions. 
At the same time it may be profitable, and not without interest, to institute com- 
parisons with one or two others of the best-known regions. The phenomena are 
so complex that even tentative comparisons may be of use in pointing out some of 
the lines along which it will be most profitable to work, or in calling attention to 
certain lacunae in our knowledge or to features which have been overlooked and 
which may demand investigation. 
From a general survey of the literature it is clear that it is far easier to find 
well-described regions which differ radically from the Roman than to find those 
which resemble it. Thus the Christiania Region, so well-known through the classic 
researches of Brogger, that of Madagascar, recently described by Lacroix, and that 
of eastern Africa, known through the studies of Prior and others, while they resemble 
the Roman in the dominantly alkalic character of the rocks, are strikingly dissimilar 
in that they are dominantly sodic. Most of those around the borders of the Pacific 
Ocean, those of Great Britain and Iceland, those of Hungary and the Grecian 
Archipelago are all much more prominently calcic and with soda dominating potash, 
and in some cases with highly quaric magmas. But it is needless to cite more 
examples of the dissimilar ones. 
In certain respects the Bohemian Region, so ably described by Hibsch, and 
that of the Eifel and Laacher See, of the rocks of which satisfactory analyses are 
singularly few, resemble ours. Leucitic rocks are present to some extent and the 
magmas are for the most part highly alkalic, but here also the soda dominates the 
potash, giving rise to many highly nephelinic and orthoclase-poor types of rock. 
The only sufficiently known region which is strikingly analogous to the Roman 
one is that of central Montana, the knowledge of which we owe to the labors of 
Weed and Pirsson, and the general characters of which have recently been summar- 
ized by the latter.* 
While the geologic occurrence of the rocks of this is very largely intrusive, in 
the form of laccoliths and stocks with their attendant dikes and comparatively 
* L. V. Pirsson, Am. Jour. Sci., XX, 1905, p. 35. 
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