SCHTSTOSE ROCKS OF NORTFTERX DONEGAL. 



231 



chlorite," " crystalline limestone, with garnet," " calcareons schist, 

 with some quartz and mica." 



Some account of the distribution and relations of the above 

 groups will be given in the following paragraphs. 



Section from Londonderry to Du naff Head (fig. 5, p. 235). 



This line passes from S.S.E. to X.X.W. across a band of about 20 

 miles in breadth, striking to the S.^\ The Lough-Foyle series occu- 

 pies the south-eastern side of the zone, and is 13 miles broad. The 

 "remainder of the breadth is covered by the upper part of the crystal- 

 line group. 



Drumahoe Quarry (two miles E.S.E. of Derry). — The quarry- 

 section is now partly obscured by debris, and rock is better seen in 

 and near the river-bed, where a fine-grained limestone, 3 ft. thick, 

 and some slaty bands, dip E.S.E. at 80°. I^ear the water-mill, a 

 little lower dovni the stream, are fine-grained bluish-grey schist and 

 altered quartzose grit, rather contorted, dipping at a low angle to 

 the l^.W. The schist is of the peculiar type so characteristic of the 

 hypometamorphic rocks of Anglesey. At first sight it might be taken 

 for shale or slate ; but looking more closely we see the surfaces of 

 the laminae glistening like the folia of a true mineralized schist. 

 This view is confirmed by the microscope. Prof. Bonney considers 

 the rock a schist of a late type, in which the fragmental structure 

 is still conspicuous. I call special attention to this variety, because 

 it predominates throughout the newer Archaean rocks of Ireland, 

 wherever I have seen them. 



LondondeiTy. — j^ear the east end of the Eoyle Bridge there is a 

 large quarry of normal hypometamorphic schist, dipping jN".W. at 

 60°. On the west side of the river, in the Strand, the rock is a 

 quartzose grit, separated into thin seams by folia of mica, and is 

 highly suggestive of the " foliated grit " of Melin pant y Gwyda *, 

 in Anglesey. About two miles north of the city, east of Summer 

 House, there is a good exposure of similar schistose grit in a small 

 quarry. The beds are puckered into gentle undulations, such as 

 might be mistaken for ripple-marks. A mile and a quarter due west 

 of this section, in some large quarries on the Buncrana Road, is an 

 extensive exposure of rock which differs from the preceding only 

 varietally. The gritty materials, consisting of quartz and felspar, 

 are very clearly fragmental. The schist is very fine-grained, often 

 occurring as mere flakes in the grit, sometimes in regular seams. 

 The dip of the rocks hitherto examined west of Londonderry is 

 usuaUy jS^.N.W. 



Cashel Hill. — This elevation, four miles N.W. of the last locality, 

 is the spur which terminates Scalp ]^Iountain on the S.W. The 

 rock is of grit, arranged in thin regular seams, and little of the 

 schistose material is present. The bedding, a little contorted, dips 

 jS'.jST.W. at from 30° to 60"^. This grit passes down through gritty 

 beds with schistose interlaminations into intensely contorted satiny 

 schist, in folia so thin that, but for the micaceous lustre, we should 

 * Quart. Journ. Geol. Soc. (May 1881), vol. xxxvii. pp. 222, 234. 



