Cote & CunnincHam—On Certain Rocks styled “‘Felstones.”? 828 
are probably offshoots from the adjacent mass of granite, we may 
presume that they had in their minds the large body of evidence 
in favour of magmatic differentiation. It is noteworthy, however, 
that the Barnesmore granite becomes highly charged with biotite, 
and is obviously darkened at its junction with the Dalradian 
schists. Any magma thus modified by contact-absorption will 
naturally occur as a superficial shell, so to speak, about the 
underlying large intrusive mass; and the impure material will 
often be the first to find its way into the fissures opened in the 
surrounding rock. Its junction with the walls of the fissures may 
be clean and clear, seeing that the material which has rendered 
the magma of the dykes more basic has been derived, not 
from the walls of the dykes, but from blocks absorbed in the 
caldron far below. If, at a later date, the fissure opens again, 
along the centre of the consolidated dyke, the purer magma from 
below will have a chance of rising, and a “ composite dyke” will 
result, its centre being far more rich in silica than its margins. 
These remarks are not put forward as a theory to account for all 
similar phenomena, especially in regions with which we have no 
personal acquaintance; but it is obvious that in the county of 
Donegal, where the modification of granitic magmas by absorption 
is so clearly demonstrable in the field, the possibility of such inter- 
action must be borne steadily in mind. 
The final chemical composition, and the final products of 
crystallisation, of any dyke, or sheet, or “batholite,’’ may thus be 
related, not only to events taking place within a subterranean 
caldron, but to the whole previous geological history of the district. 
In conclusion, it will be clear that there is a wide field for 
further study among the “felstone”’ dykes of the county of 
Donegal. The question of their age naturally arises, and cannot, 
we fear, be satisfactorily settled. If the final shearing and folia- 
tion of the schists of Donegal is, as is very likely the case, post- 
Silurian, the “felstones” are at the earliest of Devonian age. 
At the same time, dykes of this type have not been seen to cut 
Carboniferous strata in Donegal; they are, moreover, as we have 
already stated, traversed by the Cainozoic dolerites. In the south 
of the county of Tyrone, there are a number of “porphyrite ” lavas 
among the Lower Devonian sandstones, presenting the characters 
of the andesitic series of the same age in southern Scotland. 
