asia aay 
XXII. 
ON CERTAIN ROCKS STYLED “FELSTONES,” OCCURRING 
AS DYKES IN THE COUNTY OF DONEGAL. By 
GRENVILLE A. J. COLE, M.R.LA., F.G.S., ann J. A. 
CUNNINGHAM, A.R.C.S8c.1., B.A. (Prares XIX. anp XX.) 
[Read Aprin 25; Received for Publication May 1; 
Published Junz 30, 1900. ] 
Amone the schists and quartzites of the north-west of Ireland, 
which are grouped together provisionally as Dalradian, a number 
of igneous dykes occur, which are divisible into three main 
series. Tirst, there are the basic rocks that were intruded into 
the sediments prior to the period of their disturbance and meta- 
morphism. These occur mostly as sheets and sills, but occasionally 
eross the bedding-planes, or swell out as larger intrusive masses. 
Their relation to the metamorphic rocks, and the contortion they 
have undergone, are excellently shown on sheet 11 of the 1-inch 
map of the Geological Survey of Ireland, in the region west of 
Rathmullan. These rocks are now mostly in the condition of 
epidiorites and hornblende-schists, and have probably supplied 
many of the dark inclusions of amphibolite that are found in the 
intrusive gneiss of Donegal. 
The second series of dykes is described in the publications of the 
Survey by the convenient field-term of “felstone.” The mapping 
of this system of dykes shows that they belong to some period 
later than the general folding. One of them, for instance, in 
sheet 11, cuts right across the contorted epidiorites. Dr. Hyland 
has noted,’ moreover, that these later intrusive masses do not 
show signs of mechanical deformation. At some points, as in the 
south-west of sheet 24, the ‘‘felstones’”’ have been faulted; but 
_ +See G. Cole, ‘‘ Metamorphic Rocks in Tyrone and Donegal,’”’ Trans. Roy. Irish 
Acad., vol. xxxi. (1900), (in the press). 
* Geol. Survey of Ireland, Mem. Sheets, 3, 4, 5, 9, &c., p. 143. 
