190 THE ROMAN COMAGMATIC REGION. 
Geological Survey show an average for K 2 O of 2.61, that for BaO being 0.094. 
This figure for K 2 O is distinctly below the average for igneous rocks as a whole, 
while that for BaO is somewhat above the average already given. Also the rocks 
of the country, so far as known, are relatively high in soda, dosodic and sodipotassic 
subrangs predominating, and nephelite being quite common. But it is notable that 
the highest figures for K 2 O, 0.32 and 0.30, are reported for the "leucite-basalts" of 
Byerock and El Capitan, and that in the "oli vine-basalts" and " monchiquites " the 
percentages of BaO are expressed in a few hundredths of i per cent. 
In greater detail it will be found that for any region the variation of BaO corre- 
sponds in a general way with that of K 2 O, though there are exceptions to this in 
all. From the facts above we may infer, at least provisionally and with due 
recognition of the insufficiency of our data, that there is some correspondence 
between the K^O and the BaO in igneous rocks, and that the two go hand in 
hand more or less, the detailed relations varying with the special characters of the 
region. 
But while such a general correspondence is thus indicated, it can not be denied 
that there are exceptions and that there does not seem to be any strict proportionality 
between the amounts of the two constituents, at any rate when different regions 
are considered. This is illustrated by the figures for the Roman and the Central 
Montana Regions, and among the analyses of the rocks of the United States, New 
South Wales, and the Roman Region cases can be found with quite high figures for 
BaO, connected with relatively low ones for K 2 O. 
It would seem, therefore, that the distribution of BaO, while almost certainly 
connected in some way with that of K 2 O, is dependent also upon more general 
magmatic or regional characters. It is uncertain as yet whether the correspondence 
between the two, assuming this to exist, is due to some relation between potas- 
sium and barium as elements, to the similarity of their behavior in regard to the 
physico-chemical processes involved in the formation or differentiation of igneous 
rocks, or finally to a mere coincidence, the correspondence being brought about by 
chance segregations in the earlier stages of the formation of igneous magmas in the 
body of the earth. 
The present case seems to be analogous to the predilection of ZrO 2 for sodic 
rocks and that of Cr 2 O 3 for those high in magnesia. Exceptions to both of these 
are known, in the sense that highly sodic rocks are found with little or no ZrO 2 and 
highly magnesic ones in which Cr 2 O 3 is absent. But the general correspondence 
between these pairs of constituents, as well as between others which might be men- 
tioned, is now generally recognized in petrology. 
In view of the common and widespread occurrence of such correspondences, 
the exceptions should not be held to invalidate the final conclusions as to the distri- 
butions. They indicate rather that the relations are more complex than is now 
apparent, and that the distributions of the rarer elements, as well as of the more 
common ones, among igneous magmas or rocks are the resultants of many factors. 
