150 THE ROMAN COMAGMATIC REGION. 
the abundance of dopotassic subrangs. The exceptions to this are for the most part 
confined to the rarer rock types, the only important one being the rather common 
types of phlegrose (1.5.1.3), which will be discussed later. 
Another of scarcely less importance is the dominance of alkalis over salic 
lime, that is lime which enters into the normative feldspars. This is clearly shown 
by the abundance of rocks falling in peralkalic or domalkalic rangs, the alkali- 
calcic rangs being few and mostly represented by rare types. 
As a general thing, also, the molecules of ferrous oxide and magnesia together 
surpass those of total lime, and they invariably do those of femic lime, while molec- 
ularly magnesia is usually higher than ferrous oxide, though there are some notable 
exceptions to this last statement. Similarly titanium is invariably higher than 
phosphorus and baryta higher than strontia, these being the almost universal 
relations. 
To sum up the absolute chemical characters of the region then, we may say 
that as regards the main constituents, silica is moderately high to moderately low 
and mostly deficient; alumina uniformly high and with a remarkably narrow 
range ; ferric and ferrous oxides and magnesia low and lime rather high ; magnesia 
usually dominating ferrous iron, and both usually dominating lime and always 
femic lime; the total alkalis high, with soda usually low to moderately high, and 
potash high to very high; potash dominating soda and the alkalis together domi- 
nating salic lime; as regards the minor constituents, titanium and barium are 
high and phosphorus and manganese low, these being always present; zirconia, 
strontia, and the oxides of cerium, etc., are usually present, but only in traces; sul- 
phur and chlorine in varying but small amounts; and chromium, nickel, and cop- 
per are absent. 
SERIAL CHEMICAL CHARACTERS. 
The most obvious of these is that as silica falls the iron oxides, magnesia, lime, 
titanium, and phosphorus rise, while the total alkalis fall, alumina keeping fairly 
constant. Except as regards alumina this is the usual relation, obtaining not only 
for comagmatic regions (petrographic provinces) , but for igneous rocks as a whole. 
It finds its expression in the division of classes in the quantitative system based 
on the relative amounts of salic and femic minerals in the norm. 
Within this general variation some more special ones of interest may be 
observed. As silica decreases, or as the femic components increase, the amount of, 
potash relative to soda increases on the whole. There are some exceptions to this, 
but the general law for this region seems to be amply substantiated by the great 
majority of the analyses. It is, however, of great importance to note that, with 
the exception of the persalic rocks in which soda is high, this relative increase in 
potash is brought about rather by increase in the potash relatively to the silica and 
the femic constituents than by decrease in the soda, the amount of this varying only 
within rather narrow limits. It is also shown by the general fact that the potash 
is higher relatively to the soda as the amount of lenic minerals in the norm or of 
