244 Cotre—On the Geology of Slieve Callion, in the County of Londonderry. 
Mr. Nolan* in 1879 made the brilliant suggestion that the granite intrusions were 
the result of the depression and remelting of the ancient metamorphic series ; the 
mass thus flowed as an igneous rock, though originally it was of metamorphic 
origin. In this he anticipated the work of Dr. A. C. Lawson} in Canada, who 
proved that the Laurentian gneiss sends off granitic veins into the ‘ later” 
Coutchiching and Keewatin series, and has even appropriated into its own body 
portions of those series. My. Nolan’s hypothesis is thus an admirable working 
one, and may yet lead to the connection of Middle Devonian granites with the 
ancient Archzean floor. 
In 1884, however, the same writer,{ in his official memoir, placed both the 
granite and the gneissic core to the west as ‘probably of pre-Cambrian age,” 
thereby doing some injustice to his previous writings. He appears to have been 
impressed with the evidence of gradation from the granite into the “ pyroxenice 
rocks,” and to have been unwilling to regard the granite series as posterior to the 
latter. We may be pardoned for accepting his earlier reading of 1878, while 
reserving our opinion as to the connection between the intrusive granite and the 
oneiss. 
The age of the basic series, voleanic and plutonic, is far more obscure. On 
Slieve Gallion itself, it has been regarded as consisting of metamorphosed Ordo- 
vician sediments; and it may even now prove to be a part of the great eruptive 
series, which characterises the Bala epoch in Ireland as well as in North Wales. 
Mr. G. H. Kinahan§ has referred these rocks, with some hesitation, to the Arenig 
epoch; and it is claimed, as we have seen, that they graduate into the series of 
schists that culminates in the Sperrin Mountains. In the combe at the head of 
the Letteran Hollow, an iron-stained bed of chert occurs at the base of the 
andesites, dipping north-east ; and other cherts, red and green, are found along the 
heematitic horizon in the neighbourhood of Tory’s Hole. I had hoped that these 
deposits, which are sometimes minutely speckled with white spots, would have 
given microscopic evidence of their origin; but sections have proved most 
unsatisfactory. The chert found near Tory’s Hole, in the Derryganard Hollow, 
contains many flakes of chlorite, and may be merely a chalcedonic replacement of 
a diabase along the zone of deposition of the iron-ores. Such a replacement is not 
unknown among basic rocks; the variolite of Ceryg-gwladys in Anglesey has even 
retained, under such circumstances, all its original structures. Similarly, the 
chert of Letteran may represent merely a phase of alteration in connection with 
* « Rocks of Tyrone,” Geol. Mag., 1879, p. 159. 
+ “Report on the Geology of the Rainy Lake Region,” Ann. Rep. Geol. Surv. Canada for 1887 (1888), 
p- 189r. 
+ Mem. sheet 26, pp. 10 and 15. 
§ ‘Economic Geology of Treland,”’ Journ. R. Geol. Soc. Ireland, vol. viii., p. 160. 
