PETROLOGY. 171 
the existence of some deep-seated "reservoir," "magma-basin," or other body 
of magma as a source, it is highly improbable that these extruded portions form 
more than a small fraction of the whole, or that their relative proportions when 
averaged are such as to give a correct idea of the true composition of the magma 
whence they are derived. It is probable that the body of extruded rock is very 
small relatively to the parent body and that it consists for the most part of only a 
certain, and not wholly representative, portion of the differentiates. 
The above conclusions would also hold good, though probably to a less extent, 
of regions where the erosion has been profound, so that the deep-seated rock bodies, 
as stocks and laccoliths, are exposed, with their satellites of dikes and sometimes 
of flows. In most cases it is probably true that here erosion has destroyed a 
very considerable portion of the masses, especially of the effusive portions, while 
again it has not progressed far enough to reveal the deeper portions, and we are 
left in the dark as to the downward extensions. 
In both cases the practical difficulty of estimating the relative masses of the 
various types is a serious one. In certain small bodies, as some differentiated dikes 
or small and deeply eroded laccoliths, as that of Shonkin Sag, exceptionally favor- 
able conditions may almost entirely overcome the inherent difficulties, and very 
reliable estimates can be made. But in the great majority of cases the difficulties 
are almost or quite insuperable, and we must perforce remain content with such 
approximate estimates as the erosion and exposures permit. 
But notwithstanding the impossibility in most cases of arriving at perfectly 
satisfactory or trustworthy data, the problem of the composition of the original 
magma is of such interest in the study of comagmatic regions, and has such impor- 
tant bearings on the phenomena of differentiation, that an attempt should be made 
to determine it wherever possible, even though the results arrived at must be 
regarded as of no very high order of accuracy and provisional in their character. 
As such observations increase in number and cover more kinds of comagmatic 
regions or differentiated bodies, their value will increase, even though they be not 
absolutely accurate, since in any case they will probably point out the directions 
in which the processes of differentiation take place, if not their extent. We 
shall at least gain some general knowledge, though all the details are unknown 
to us. 
In the case of the Roman Region, as much of it is as yet unmapped geologically, 
and in any case the thicknesses of the various flows are quite unknown for the most 
part, as well as many of them certainly covered beyond our observation by later 
flows and tuffs, the possibility of detailed knowledge of relative masses is reduced 
almost to a minimum. The only basis on which I can estimate them at present 
are my general observations, and the published statements of others, as to the 
relative abundance of the various types which are accessible to study, the relative 
areas of the different districts being also taken into consideration. We shall begin, 
therefore, with a general examination of the quantitative relations of the different 
