148 THE ROMAN COMAGMATIC REGION. 
rocks of a region may have the common feature of high magnesia, or be uniformly 
quaric (siliceous) and with lenads (leucite and nephelite) quite absent, and so on. 
These may be called the absolute characters, in contradistinction to the serial ones 
to be mentioned presently. Superposed on these absolute characters, if one may 
so express oneself, are the characters which may be called serial, and which consist 
in a definite variation in the amounts of one or more constituents concomitant with 
a variation in one or more others, which variations may be either in the same or 
opposite direction. Thus it is now well established for many regions, and as a 
general (though not invariable) character of igneous rocks, that the alkalis and 
alumina increase with increasing silica, while iron, magnesia, and often lime decrease. 
Or again, the relations between potash and soda, or between magnesia and ferrous 
oxide, may vary with variations in the silica. 
Assuming the derivation of the rocks of any given region from a common 
magma by differentiation, it would seem that the absolute characters are dependent 
on the composition of this, while the serial characters are due to the chemical changes 
brought about by the processes of differentiation, the absolute characters persisting 
in spite of these. For these reasons it would be advisable to keep clear the 
distinction between the two as far as possible. 
ABSOLUTE CHEMICAL CHARACTERS. 
The range of silica is considerable, from 62 to 45 per cent, while if the relative 
masses of the various rock types are taken into consideration, the range in the 
most abundant types would be only from 56 to 47. In general terms, therefore, 
the silica may be said to be medium to rather low and with a rather narrow range. 
Alumina is generally high and its most marked characteristic is its very narrow 
range in most of the analyses, only from 17 to 20 per cent. In only five is it higher, 
and then but slightly so, from 20.8 to 21.4, while in five others it is lower, from 
1 6. 6 to 12.7, these ten being mostly of rather rare types. 
The oxides of iron and magnesia are almost uniformly low, and the range 
comparatively small, with a few exceptions, though not as narrow as that of alumina. 
Lime, in contradistinction to the other bivalent oxides, is almost uniformly quite 
high, and with a very considerable range. Soda, on the other hand, is rather low, 
and while the total range is considerable (from i .o to 7.2), it is only from i .5 to 
3 . 5 in the majority of the analyses. Potash, like lime, is distinctly high, and with 
a very considerable range, from 3.7 to 11.3, the higher figures being almost the 
highest ever recorded for this constituent in igneous rocks, although the range of 
the majority is only from 6.4 to 9.6 per cent. 
Turning to the minor constituents, we find sufficient data for many of them to 
permit of some general statements. Titanium dioxide is almost uniformly high, 
ranging from 0.3 to 1.4, and in most of the rocks falling between 0.6 and 1.2 per 
cent. As this constituent was determined in most cases by the very reliable colori- 
metric method, the figures for it may be regarded as more trustworthy than is 
