168 
LDDINGS. 
dall basin, the extreme forms differing widely from one 
another. These extremes or end results of the differentia¬ 
tion of a particular body of magma are usually of smaller 
volume than the earlier eruptions at that center, and also 
constitute less common rocks, since differentiation has not 
advanced to the same extent in all localities. Moreover, it 
is to be expected that the end products of volcanic activity in 
a particular locality, which are of small volume, will appear 
as the last eruptions at the surface, or may in some cases 
never reach the outside of the volcano. In the first instance 
they will be among the earliest rocks to suffer from erosion, 
and, being of small volume, will soonest disappear, leaving 
only occasional remnants of their former existence. The 
portions of them which remain beneath the surface and are 
exposed by erosion may occupy relatively small spaces among 
the earlier rocks and will most frequently take the form of 
dikes, though it is not to be imagined that this is the only 
form in which they may occur. It is to these exceptional 
varieties of rock that Rosenbusch has applied the term 
“ ganggesteine ” or “ dike-rocks,” because, as he states, they 
are not found in any other mode of occurrence.* 
All other rocks in certain places constitute dikes; but since 
they also occur in other ways they are not included in the 
particular group of “ ganggesteine.” It follows, therefore, that 
if the end results of differentiation are found in other modes 
of occurrence than as dikes they cease to belong to the 
group just named. 
The application of the term “ ganggesteine ” to a very small 
number of all of the rocks known to occur in dikes appears 
to the writer to have been unfortunate, since it is not at all 
distinctive, its sole claim resting on our ignorance of the 
complete occurrence of the rocks to which it has been ap¬ 
plied. It is consequently, as Rosenbusch himself realizes, 
in the nature of a vanishing term, whose rate of disappear¬ 
ance is proportional to our advance of knowledge. 
Rosenbusch (H.) Mikroskopisclie Physiograpliie der Massigen Ge- 
steine. 8°. Stuttgart, 1887, p. 277. 
