On the Laurentian Rocks of Donegal and of other parts of Ireland. 247 
The above series has a thickness of over 2,000 feet, and is abruptly truncated at 
the margin of the old eneissose series between the entrance to Barnesbeg Gap and L. 
Greenan. To this I shall have occasion to return. Both above and below the 
Lough Salt beds there is a great series of schists, quartzites, limestones, with beds 
of hornblende schist, passing into diorite, which likewise terminate at the margin 
of the old gneissose series ; the general dip being 8. EH. 
(2.) The Laurentian Gneissose Series.—The Donegal gneiss has been correctly 
described by Dr. Haughton, and Mr. R. H. Scott as really a metamorphosed strati- 
fied formation. They also show that it contains beds of crystalline limestone or 
marble, with which several minerals, such as sphene, idocrase, and garnet, are often 
associated.* They also point out that along with the foliated and bedded gneiss, 
there is found a massive granite, in which planes of foliation are not apparent. 
This variety occurs at Dunglow and elsewhere. It is uncertain whether or not 
this latter variety belongs to the gneissic series. From what I saw of it, I am 
inclined to regard it as the more deeply seated portion of the original great forma- 
tion, in which the metamorphic action has given place to actual fusion. Leaving 
this question (which is not material) it may be stated that, as we ascend in the series, 
the foliated character becomes more apparent, and the upper beds which occur along 
the south-eastern margin are largely interstratified with hornblendic and micaceous 
schists, giving rise, as in the hills about the eastern entrance to Barnesbeg Gap and 
Crockmore, to successive ridges, formed by the solid beds of gneiss divided from 
each other by beds of softer schist. 
The typical characters of the gneissose series are well shown in Barnesbeg Gap ; 
also, in the hills bordering the Bulhalla and the Owenwee rivers, and in the valley east 
of Doochary bridge, as well as in many other sections. The rock consists of red 
orthoclase, often in large crystals porphyritically distributed, a little grayish oligo- 
clase, green and black mica, and quartz. It is generally distinctly foliated, though 
massive, and is traversed by numerous veins and dykes of pegmatite, consisting of 
red orthoclase and quartz, with very little mica. Seen on a large scale, the pre- 
dominating colour is light red, but there are places where this rich colouring fades 
into gray. 
The upper members of this great series, as far as they are exposed to view 
before being concealed beneath the Lower Silurian beds, consist of numerous inter- 
stratifications of red and gray gneiss, hornblendic and micaceous schist with garnets, 
and at rare intervals bands of crystalline limestone, with sphene, idocrase, and 
garnet, or with pseudomorphs of idocrase after garnet, as pointed out to me by 
Dr. W. Frazer.t These beds are laid open along the road from Glenlehene bridge 
* Dr. Frazer, M.R.J.A., accounts for the peculiar form of the idocrase crystals as found at Annagarry 
and elsewhere, which is that of garnet, by suggesting that the crystals are pseudomorphs after garnets. 
{ Sphene, idocrase, garnet, &c., are recorded by M. J. Durocher as abundant amongst the beds of lime- 
stone of the old gneiss (Urgneiss) of Sweden, Norway, and Finland, which is presumably of the same age 
as that of Scotland and Donegal.—“ Ann. des Mines,” Tome 15, 4° Ser., p. 181. 
