TDK ARCHAEAN Ro< t'J 



western parts of Mayo and Galway, and the two areas are almost 

 connected by a narrow tract of the same rocks, which runs from 

 soul li -west to north-east for a distance of 60 miles, extending from 

 near Castlebar through the Ox Mountains to Hamilton in Lvitnm. 

 So far as yet known fundamental gneiss is only exposed in 

 Tyrone, the gneisses of the western areas being all intrusive 

 gneissic granites. There is no great fault or thrust-plane separating 

 unaltered Hebridean from later schists, as in the north-west of 

 Scotland ; or if such a plane exists it must lie outside the north- 

 west coast of Donegal. Neither is there anything to correspond 

 with the Torridonian or with the Moine schists of Scotland. 



(1) Fundamental Gneiss. In Tyrone this forms a long 

 narrow tract of hilly country extending north-east from near 

 Carrickraore to Slieve Gall ion, about 18 miles, and having a 

 maximum width of 3 or 4 miles. Sir A. Geikie has described it as 

 " an undoubted core of Archaean gneiss, resembling in all essential 

 characters the Lewisian rocks of the north-west of Scotland." lt 

 More recently its position has been discussed by Professor G. A. J. 

 Cole, who also regards it as the oldest local rock and mentions 

 the occurrence of a much later intrusive granite, which includes 

 fragments of the gneiss and of the schists which flank the ridge. 14 



(2) Donegal Schists. The gneiss of Tyrone is surrounded 

 by a group of coarse volcanic agglomerates tuffs and lavas which 

 pass up into green chloritic schists and silvery mica-schists like 

 those of the Grampian Series. The lavas are chiefly basic and are 

 often plainly amygdaloidal and on the north-west side of the ridge 

 are singularly fresh and unaltered, a condition which Sir A. Geikie 

 attributes to the protection afforded them by the ridge during 

 the time of great pressure from the south-east The tuffs have 

 suffered much more from deformation and are now silky green 

 chloritic schists, but they seem originally to have been derived 

 from the same magma as the lavas (op. cit. p. 42). 



Farther to the north-west, in Donegal and in Londonderry, these 

 volcanic rocks are succeeded by quartzites and black schists with 

 bands of crystalline limestone. A similar assemblage of sedimentary 

 and igneous rocks is found in Galway, Mayo, and the Ox Mountains. 

 These of Slieve Gamph and the Ox Mountains have been recently 

 described by Mr. M'Henry, who considers the descending succession 

 to be as follows : 15 



4. Quartzite passing down into a sheared conglomerate which i often 



a " Boulder-bed." 



3. Limestone, often altered to whitish marble or greeu ophicalcite. 

 2. Black shales and graphitic grits. 

 1. Pebbly grits with bands of black shale or schist. 



