DEVONIAN AND OLD RED SANDSTONE SYSTEM 237 



stratigraphical horizon a thickness of about 9000 feet must have 

 thinned out southwards and have been overlapped by the beds 

 containing the Achanarras fauna. Such an overlap had been 

 inferred on other grounds by Sir A. Qeikie many years ago, and 

 on tliis view it is only the highest part of the Middle Old Red 

 Series which occurs in Nairn and Inverness and in the outliers 

 which are found in Aberdeen. 



This northern basin includes also some tracts of Upper Old 

 Red, which rests with a marked unconformity on the Middle 

 Series. From 600 to 800 feet of red and yellow sandstones can be 

 seen. They occupy a tract of some length in Moray and Nairn, 

 and outlying patches occur at Tarbat Ness, round Dornoch, at John 

 o' Groats and Dunnet Head, and again in the Island of Hoy. 



3. The North of Ireland 



As already mentioned, several tracts of Lower Old Red occur in 

 the north of Ireland, and others may be concealed beneath the 

 newer strata. One such tract occupies an area about 30 miles 

 long by 10 wide, between Lough Erne and Pomeroy in Tyrone. 

 " It consists for the most part of dark-red and purple conglomerates, 

 often coarse and massive, and of purple pebbly and fine-grained 

 sandstones, . . . with sandy shales. The pebbles in the con- 

 glomerate, which vary from the smallest size up to blocks over a 

 foot in diameter, consist of purple felstone, grits, schists, and 

 quartzite. Of all these the felstone pebbles are by far in greatest 

 proportion," and have been derived from the breaking up of 

 contemporaneous lava streams, some sheets of which still remain 

 interbedded with the conglomerates. 20 



The second area is on the east coast of Antrim between 

 Cushendall and Cushendun. Here again are conglomerates at the 

 base, consisting mainly of quartz pebbles ; above these is a series of 

 red and brown sandstones succeeded by a breccia composed of large 

 blocks of quartz-porphyry ; the whole is estimated to be 5000 feet 

 thick. This tract forms a link between the Tyrone area and the 

 Lower Old Red of the Kintyre promontory. 



Another tract of similar material has been found in Donegal 

 between Lough Swilly and Mulroy Bay. This consists of a basal 

 conglomerate .succeeded by chocolate-coloured sandstones and shales 

 with some pebbly beds, and the materials are all derived from the 

 surrounding quartzites and schists. The thickness seen is about 

 800 feet. Nothing corresponding to the Upper Series has been 

 found in the north of Ireland, though it undoubtedly occurs in 

 the southern part of the country (see p. 211). 



