92 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
to the portion of the magma from which they are derived. ' Professor Brogger 
believes that among the basic eruptive rocks of Gran in the Christiania dis- 
trict, one and the same magma has in the bosses solidified as olivine-gabbro- 
diabases, and in the dykes as camptonites, bostonites, pyroxenites, horn- 
blendites, and more acid augite-diorites.^ 
Various opinions have been propounded as to the cause or causes of this 
so-called differentiation, but none of them are entirely satisfactory. We 
must await the results of further exploration in the field and of continued 
research in the laboratory. 
What appears to have taken place within a subterranean molten magma 
which has been propelled into the earth’s crust as a boss or laccolite, with 
or without a connected system of dykes, may possibly be made to throw 
some light on the remarkable changes in the characters of lavas successively 
erupted from the same vent during the continuance of a volcanic cycle. 
Whether or not any such process of differentiation can be proved to take 
place within a subterranean volcanic reservoir, the sequence of enrpted lavas 
bears a curious resemblance to the order in which the constituents of some 
largo bosses succeed each other from margin to centre. The earliest lavas 
inay be of an intermediate or even basic character, but they generally tend 
to become more acid. ISTevertlieless alternations of basic and acid lavas 
which have been noted in various districts would seem to show that if there 
be a process of differentiation in the magma-basins, it is not regular and 
continuous, but liable to interruption and renewal. The return to basic 
eruptions, which so often marks the close of a volcanic cycle, is likewise not 
easily explicable on the supposition of continuous differentiation. 
Where no sensible evidence of differentiation is traceable in the general 
body of a large intrusive mass, indications that some such process has there 
been in progress are perhaps supplied by the more acid dykes or veins, 
and the sd-called “segregation veins,” which have been already alluded to 
as traversing large intrusive masses. Though these portions differ to a 
greater or less extent in texture and composition from the main substance 
of the boss, the differences are not such as to prevent us from regarding them 
as really parts of the same parent magma. The veins, which are more 
acid than the rock that they traverse, may he regarded as having emanated 
from some central or deeper-seated part of a boss, which still remained lluid 
after the marginal or upper portion had consolidated sufficiently far to be 
capable of being rent open during subterranean disturbance. But that the 
mass, though coherent enough to be fissured, still remained at a high tem- 
perature, may be inferred from the general absence of chilled edges to these 
veins. The evidence of differentiation supplied by “ segregation veins ” has 
been referred to in the case of Sills. 
The study of the petrographical variations in the constitution of large 
eruptive bosses has a twofold interest for the geologist. In the first place, 
it affords him material for an investigation of the changes which a volcanic 
magma undergoes during its eruption and consolidation, and thereby pro- 
* Quart. Jown. Geol. Soc. 1. (1894), p. 35. 
