50 REPORT—1863. 
of them traceable in other parts of the county, but it is only here that hey 
assume a development of such importance. 
On passing west from Buncrana towards Dunaff Head, through the gap of 
Mamore, it is found that, as we approach the granite at Urrismenagh, the 
dip of the beds increases from 45° to nearly absolute verticality. The granite 
of Urrismenagh does not present many features of interest, as the rocks in 
immediate contact with it are quartzose, and therefore unlikely to yield 
accidental minerals. 
The rocks lying between Rathmullen and Milford are similar to those 
which have been already described as occurring in Innishowen; however, 
the granite of Kindrum in Fanad deserves a special notice, as it is somewhat 
remarkable in its character, resembling the variety which is found at Ardara 
and also in the island of Arranmore, as will be noted hereafter. It is, in 
general, white, and contains a large quantity of black mica and of sphene ; 
but there is a considerable quantity of a reddish granite found disseminated 
through it. The nature of this granite differs materially from that of the 
typical granite of the central valley. It is a remarkable fact, that all the 
localities in which this white ‘‘sphene-granite ” (as we call it, from the great 
abundance of that mineral in it) occurs are situated at a distance from the 
central granitic area. 
At Glen, which is situated close to the head of Sheephaven, and at the 
northern extremity of the lake of that name, the central granite ends rather 
abruptly ; it is flanked on the east side by a very peculiar, highly micaceous 
gneiss, called in the district “black granite.” Of this rock there are two 
varieties, one of which contains a reddish feldspar, the other a grey one, this 
latter exhibiting the strie of an anorthic feldspar. The granite itself is very 
similar in its appearance to that which occurs near Doocharry Bridge in the 
Guibarra valley, and is characterized by the presence of the two feldspars, 
orthoclase and oligoclase, the orthoclase being of a flesh-red colour. 
Close to Glen, at Lackagh Bridge, on the road to Creeshlagh, we meet with 
a very remarkable illustration of the nature and relations of the rocks 
throughout the whole county. A series of quartzites and hornblende slates 
are met with, the latter passing gradually into syenites. Their strike is 
E. 5° N., and their bedding vertical. They are traversed, along the strike, 
by a series of veins of granite, which rarely cross a bed, but still preserve the 
character of veins, not of beds, as they are decidedly not lenticular. 
Appearances precisely similar to this have been observed at Toberkeen, 
and at Stackamore, about half a mile north of Leabgarrowin Arranmore. The 
bearing of this fact on the geology of the county will be again referred to. 
The chief point noticeable about the neighbourhood of Dunfanaghy is the 
extreme development of a highly crystalline syenite, containing a large 
proportion of titaniferous magnetic iron. The octahedral crystals of this 
mineral are very noticeable on the weathered surface of the rock. The best 
specimens are obtainable close to M‘Swyne’s Gun, at Horn Head. The mag- 
netite also occurs in a rock composed mainly of black mica—an occurrence 
very similar to that which it has in some parts of Norway. 
The quartz-rock of Horn Head is highly characteristic, being eminently 
flaggy in its nature, and splitting into long fluted blocks resembling Sigillarie 
at first sight. This variety of the rock contains a considerable quantity of 
feldspar disseminated through it in grains, as if ina porphyry. The same 
sort of rock is found also at Crohy Head. There is another variety of quartz- 
rock, which contains mica in large quantity as extraneous element, and is 
found to occur extensively in Arranmore. These two varieties of quartz-rock 
