ON THE CONSTITUTION OF THE GRANITES OF DONEGAL, 51 
have been noticed as occurring both in Norway and in Canada. In the former 
locality Keilhau especially refers to the feldspathic variety as being easily 
disintegrable into sand. It is a remarkable confirmation of this, that on the 
flat summit of Muckish Mountain, which is itself composed of this quartz- 
rock, there is a very large deposit of siliceous sand in a condition of nearly 
absolute chemical purity. 
The other variety is mentioned in the Report of the Geological Survey of 
Canada before referred to, in the following words :—*The quartzites have some- 
times the aspect of sandstones, and at other times lose their granular texture 
and become a vitreous quartz. Not unfrequently the quartzite is thin-bedded 
and even schistose in its structure; and it sometimes holds a little mica, 
passing into a variety of mica-schist.” In Mr. MacFarlane’s paper, he alludes 
to both varieties as occurring in Norway. 
There is found throughout the county a considerable quantity of highly 
crystalline metamorphic limestone, which is usually of a bluish colour. No 
traces of fossils have as yet been discovered in it, although at one locality, 
Culdaff, concretions have been found which have been supposed by some 
persons to be the half-obliterated remains of corals. We do not see any 
reason for attributing an organic origin to them. The limestone-beds which 
occur in immediate proximity to the granite, but not in actual contact with 
it, are converted into white marble. Those which are found in contact with 
the granite have undergone a further alteration, several minerals having been 
generated in them. Among these we may mention garnet, idocrase, epidote, 
tremolite, &c. 
Tn almost all the localities where the limestone occurs in the granite, we 
find, in immediate contact with the limestone, a rovk which we have termed 
“‘sphene-rock,” as it is characterized by the great abundance of that mineral, 
It consists of orthoclase, green pyroxene, and quartz; and in it we have 
discovered minute crystals of blue apatite, and in one locality (Glenleheen) a 
great abundance of white scapolite. 
Dr. T. Sterry Hunt, on seeing our sphene-rock, recognized it at once as an 
old friend. He says of it, «‘ Associated with the Laurentian limestones, there 
are frequently found beds of a coarse-grained rock made up of white feldspar 
and dark-green pyroxene with brown sphene, and occasionally with quartz. 
The feldspar is found to be orthoclase.” The rock is also well known to the 
quarrymen in Canada as the next bed to the limestone. 
Although this seems to point to a certain similarity between the limestones 
of Donegal and those of the Laurentian rocks of Canada, we should say that 
we have not been able to discover any deposits of either apatite or graphite in 
appreciable quantity. These minerals are stated to accompany the Laurentian 
limestones, and to be an important feature in the rocks belonging to that series. 
In order to examine the granite more thoroughly, we have crossed it several 
times, and have found the results of all the sections to be nearly identical. 
The granite is, in general, fine-grained ; but there is one important district to 
which this statement will not apply, viz. that on the west coast, which 
comprises Dunglow, Annagary, and Arranmore Island. In this part of the 
county the rock is coarsely crystallized; and it is a remarkable fact that the 
same district is further characterized by the appearance in it of a series of 
joint-planes, which do not coincide with those observed in other parts of the 
county. Attention has already been drawn to this fact by Professor Haughton, 
in a paper in the Quarterly Journal of the Geological Society of London, vol, 
xviii. p. 405. 
In the very heart of its area, the granite, judging from hand-specimens, is 
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