On the Laurentian Rocks of Donegal and of other parts of Ireland. 245 
IPATRMO IL, 
PRELIMINARY STATEMENT. 
With a view of determining the geological age and relations of the old gneiss of 
Donegal, I made an examination of the line of boundary between it and the meta- 
morphosed Lower Silurian beds, as very well laid down on Griffith’s Geological Map 
of Ireland, during the early part of last summer, in which examination I had the 
assistance of Mr. R. G. Symes, F.G.S., Mr. S. B. Wilkinson, and, for a part of the time, 
Mr. J. Nolan, officers of the Geological Survey.* We commenced our observa- 
tions at Fintown, west of Stranorlar, and continued them, with but slight intervals, 
all the way to Lough Salt (correctly L. Alt) near Glen, at the north end of the 
gneissic tract. We also examined the N.W. boundary, from Creeslough to Dunlewy 
and Gweedore, at several points, and also made a complete transverse of the gneissie 
district from the Atlantic coast at Dunglow, by Doochary, Glenleheen Bridge, and 
Fintown, to Letterkenny. This exploration— extending over ten or eleven days— 
enabled us to arrive at the following conclusions :— 
First—That the gneissose series of Donegal is unconformably overlaid by 
metamorphosed Lower Silurian beds along the eastern and southern boundaries. 
Second.—That the similarity in physical characters of the gneissose series, and 
indentity of stratigraphical position, with the “fundamental gneiss” of parts of 
Sutherlandshire and Rosshire, affords a presumption that, like the latter, it is of 
Laurentian age.t 
Third.—That the north-western boundary of the gneissose series is a large down- 
cast fault which ranges from the entrance to Barnesbeg Gap, near Creeslough, to 
the eastern base of Errigal at Dunlewy ; and that the boundary at Gweedore is pro- 
bably also a fault transverse to the former, 
Fourth.—Yhat there is no evidence of the presence of any beds representing the 
Cambrian series ; and, therefore, that the unconformity above referred to represents 
a great hiatus, such as occurs in the series of formations in the districts of Ben 
Arkle and Foinaven in Sutherlandshire, where the Lower Silurian quartzites rest 
directly onthe fundamental gneiss: the Cambrian sandstones having here disappeared. 
Description oF Sxuctions, &c. 
I now proceed to give some account of the two great series of unconformable 
formations of North-west Donegal. This account need only be brief as previous 
authors have so fully described them. 
* This examination was official, and made with the assent of the Director-General. 
+ It will, probably, always be impossible to demonstrate that the British rocks lying below the Cambrian 
beds are of the same age, and strictly representative of the Laurentian beds of Canada, but I agree with 
” 
Professor Ramsay in the opinion, “ that the presumption that they are of Laurentian age is very strong. ”— 
Phys. Geol. of Gt. Britain. 5th Edit. 
282 
