434 
THE TERTIARY VOLCANOES 
BOOK VIII 
This solvent action may serve to explain some of the irregularities of the 
granophyre intrusions. According to the same observer, such irregularities 
are found “where the granophyre sheet and its encasing basalt-sills are not co- 
extensive, or again where the two basalt-sills separate, owing to one of them 
cutting obliquely across the bedding. In the latter ease, which is not 
common, the granophyre follows one of the basalt-sills, necessarily parting 
from the other. When one of the two guiding basalt-sills dies out, the 
granophyre may still continue, following the sill which persists. If the 
latter also dies out, while the granophyre is still in some force, the acid 
magma seems to have been reluctant to travel beyond the limit of the basalt, 
but has drawn towards it, and the granophyre presents a blunt laccolitic 
form, which contrasts with the acutely tapering edge of a granophyre which 
dies out before reaching the limit of its basalt-sills. If, on the other hand, 
on reaching the limit of the basalt, the acid magma has been in such force as 
to be driven further, it is usually found to lose something of its regularity 
and to depart from the exact horizon which it has hitherto followed. This 
seems to happen, for instance, in the Beinn a’ Chairn sheet, which, when 
traced westward, is found to behave as a ‘ boss ’ and is obviously trans- 
gressive, having cut across the bedding of the strata so as to enter the 
limestones, where it no longer behaves in any degree as a sill. The 
district affords many examples of the tendency of intrusive masses in 
general to cut sharply across the beds when they enter a group of lime- 
stones.” 
More complex examples of acid sills are to be found where there have 
been three or more basic sheets together. The great granophyre sheet 
already referred to at Suisuish affords the best illustration of this structure. 
Mr. Marker has noticed that “ round most of its circumference there is seen 
merely a single basalt-sill passing under the granophyre. Probably there 
has been another similar sheet over the acid rock, but if so, it has been 
removed by erosion, the granophyre itself forming everywhere the surface 
of the plateau. On the southern side, however, we see that the original 
basalt must have been at least triple, or counting the uppermost member, 
now removed, quadruple. The granophyre lias forced its way in between 
the several members of the multiple basalt-sill, the intermediate ones being 
thus completely enveloped. They are evidently metamorphosed as well as 
veined by the granophyre, and when traced onward they give place to 
detached portions which, floating as it were in the acid rock, are soon lost.” 
It is seldom easy to determine where lay the vent or vents from which 
the granophyre sills proceeded. Those of the Skye platform just described 
may be chiefly concealed under some of the larger areas of the rock, such as 
the sheets of Carn Dearg or Beinn a’ Chairn. But in several places, in close 
association with the compound sills of granophyre and basalt, Mr. Harker 
has found large dyke-like bodies of the acid rock, which may with consider- 
able probability be regarded as marking the position of the channels by 
which the material of the sills ascended. “These bodies,” he remarks, 
“ either occur isolated by erosion, the sills or the parts of the sills presumed 
