246  CoLtn—On the Geology of Sheve Gallion, in the County of Londonderry. 
VII.—Conccustion. 
I may now sum up by setting down the following conclusions :— 
(i.) The series of hornblendic and pyroxenic rocks on Slieve Gallion, hitherto 
described as of metamorphic origin, includes a volcanic series of andesite-tuffs and 
vesicular and compact andesites, together with their deep-seated representatives. 
The age of this series is ‘‘ Dalradian,” using that term in its widest sense. 
(ii.) he granite, also once held to be of metamorphic origin, is an intrusive 
mass, which has absorbed some of the basic rocks, and has produced quartz-diorites 
by a process of intermingling. 
The period of its intrusion was pre-Carboniferous, and probably Middle 
Devonian, as stated by the officers of the Geological Survey. 
(iii.) The basic series west of Cookstown, including the volcanic tuffs of Beagh- 
beg, is indistinguishable from that of Sheve Gallion, and is almost certainly of the 
same geological age. 
The relations of this series to the gneiss that underlies it have yet to be satis- 
factorily worked out. The suggestion of Mr. Nolan, that the gneiss became 
remelted to provide the granite veins above it and the granite mass of Slieve Gallion, 
deserves the most careful consideration. 
(iv.) The occurrence of aplitic granites and eurites on Slieve Gallion, associated 
with varieties rich in biotite and in hornblende, and the discovery of intrusive veins 
of pure soda-orthoclase near Oritor, suggest that even the biotite in the granite 
may have resulted from the absorption of the basic series by a magma that would 
otherwise have crystallised as an aplite; and, following on this, it is urged that 
the underlying magmas of the earth’s crust may be of far simpler character than 
has been commonly supposed. Prof. Sollas’s investigations at Barnavave seem to 
point to the same conclusion. It is then suggested that plutonic rocks, as we ordi- 
narily know them, are phenomena of contact, produced in what are, comparatively 
speaking, the upper layers of the earth’s crust. 
(v.) By a combination of absorption and concomitant or subsequent differ- 
entiation, an invading igneous rock may come to occupy the place of a pre-existing 
rock, and may, in fact, represent it as a pseudomorph, the absorbed matter being 
drawn off through the molten mass to lower levels. 
Lastly, I would express my obligations to all those who have helped me, by 
advice or in the field, including Mr. J. Nolan, Mr. A. McHenry, Dr. G. J. Hinde, 
F.R.S., and Sir Wm. Lenox Conyngham, K.C.B., of Spring Hill, Moneymore ; 
and to my Wife, for the determination of many of the specific gravities recorded 
in the present paper. 
