90 
SUBTERRANEAN VOLCANIC ACTION 
KOOK I 
bosses. Hiige masses of granite, grauophyre, quartz -porphyry, felsite or 
rhyolite, represent the acid series. Intermediate varieties consist of trachyte, 
phonolite, diorite, andesite or other rock. The basic bosses include varieties 
of gabbro, dolerite, basalt, picrite, and other compounds. 
In a boss of large size, a considerable range of texture, composition and 
structure may often be observed. The rock is generally much coarser in 
grain than that of thin sills or dykes. Sometimes it exhibits a finer texture 
along the margin than in the centre, though this variation is not usually so 
marked as in sills and dykes. The rapidly-chilled and therefore more 
close-textured selvage seems to have been developed much more fully in 
small than in large masses of eruptive material. The latter, cooling more 
slowly, allowed even their marginal parts to retain their heat, and some- 
times perhaps even their molten condition, longer than small injections. 
Some influence must also have been exercised by the temperature of the 
rocks into which the eruptive material was intruded. Where this tempera- 
ture was high, as in deep-seated parts of the crust, it would allow the 
intrusive magma to cool more slowly, and thus to assume a more coarsely 
crystalline condition. The absence of a close grain round the margins of 
granitic bosses may be due to this cause. 
But a much more important distinction may he traced between the 
central and marginal parts of some large bosses and thick sills. I have 
already alluded to the fact that wlhle the middle of a large intrusive mass 
may be decidedly acid, taking even the form of granite, the outer borders 
are sometimes found to he much more basic, passing into such a rock as 
gabbro, or even into some ultra-basic compound. Between these extremes of 
composition no sharp division is sometimes discoverable, such as might have 
been expected had the one rock been intruded into the other. The differ- 
ences graduate so insensibly into each other as to suggest that originally 
the whole mass of the rock formed one continuous body of eruptive material. 
It is possible that in some cases the magma itself was heterogeneous at tlie 
time of intrusion.^ But the frequency of the distribution of the basic 
ingredients towards the outer margin, and the acid towards the centre, points 
rather to a process of differentiation among the constituents of the boss 
before consolidation. In some instances the differentiation would appear 
to have taken place before crystallization to any great extent had set in, 
because the minerals ultimately developed in the central parts differ from 
those at the sides. In other eases, the transference of material would seem 
to have been in progress after the component minerals had crystallized o\it 
of the magma, for they are the same throughout the whole intrusive mass, 
but differ in relative proportions from centre, to circumference." 
As illustrations of these features I may cite two good examples, one 
from Scotland and one from England. The mass of Garabol Hill, in the 
^ The Tertiary gabbros of the Inner Hebrides liave already been oited, and will be more fully 
described in a later chapter a.s exhibiting the heterogeneousness of an eniptive magma. 
- See Messrs. Dakyns and Teall, Quart. Jourii. Gcol. Soc. xlviii. (1892), p. 104 ; Prof. Briigger, 
op. dt. 1. (1894), p. 15 ; Mr. A. Harker, op. dt. p. 320; Prof. Hidings, Journ. Gcol. Chicago, i. 
(1893), p. 833 ; Bull. Phil. Soc. Washington, ii. (1890), p. 191 ; 1892, p. 89. 
