48 REPORT—1863. 
the property in question stands exactly upon the same footing as other physical 
properties, such as specific volume, specific heat, isomorphism, &c., which do 
not directly indicate anything with regard to the chemical properties of 
bodies, but which, after having been found to bear a relation in very many 
cases to those properties, are safely taken as guides in drawing conclusions as 
to the nature of bodies whose chemical character is as yet unknown. 
On the Chemical and Mineralogical Constitution of the Granites of 
Donegal, and of the Rocks associated with them. 
By a Committee consisting of Robert H. Scott, Sir R. Griffith, Bart., and the 
Rev. 8S. Haughton, M.D., F.R.S8., appointed at the Manchester Meeting, 
1861. 
Tue county of Donegal consists to a great extent of metamorphic rocks, 
However, in its southern portion there is a district in which strata belonging 
to the Carboniferous period are found. The granites of the county, which 
are the main subject of this report, are found in several localities. The most 
extensive appearance of rocks of this nature is in a tract whose longer dia- 
meter is about 30 miles in length, and coincides nearly exactly with the axis 
of the valleys of Gweebarra and Glenveagh, which traverses the county in a 
direction from N.E. to S.W. In Glenveagh the granitic district is confined 
to the valley itself, and is flanked on each side by other rocks ; but as soon as 
we reach the head of that glen, we find that the granite extends over the 
whole country to the westward, and forms almost the entire coast from 
Bloody Foreland to the mouth of the Gweebarra River. 
Closely connected with this tract of granite are the isolated patches of the 
same rock which are found at several points, such as Ardara, Urrismenagh 
(near Dunaff Head), and Ardmalin—all of them situated nearly along the line 
of the great valleys before mentioned. Granite also appears in small quantity 
in Fanad and Rossguill, and on the south coast of Arranmore Island. The 
granite of the Bluestack and Barnesmore Mountains, in the 8.E. of the county, 
is very dissimilar in its appearance to that of the western district ; and although 
they may be generalized in a map sufficiently under a common designation, yet, 
from internal evidence, we are not disposed to consider them to be connected 
with each other. 
The other rocks which occur in the district which comes under our notice 
are gneiss, mica slate, quartz-rock, grit, crystalline limestone, and a variety 
of syenitic* rocks, which do not differ much from each other in chemical 
constitution, although they are very dissimilar in texture. The true gneiss 
(as distinguished from gneissose granite) and the mica siate are found chiefly 
in the south of the county. 
Before we proceed to discuss the chemical composition of the granites, it 
may be well to give a sketch of the geological features of the district, as 
observed during the several tours made by the members of the Committee in 
pursuit of this investigation. On one of these Mr. Jukes, Local Director of 
the Irish Geological Survey, was kind enough to accompany us and give us the 
benefit of his valuable assistance and experience of similar rocks in New- 
foundland. 
* The term syenite is used for any coarsely crystalline rock containing, as its most im- 
portant constituent, a hornblendic mineral, associated with a feldspar, and occasionally 
with quartz or mica, or both, Some analyses of these rocks are given in Table YI. p. 62. 
