120 Swanston— Silurian Rocks of Co. Down. 



represented and in part combined with it, as several of its most characteristic 

 fossils have been detected in its highest bands 



The top of the Upper Birkhill Shales, marked by the zone of Rastrites 

 maximus, has not yet been detected in Ireland ; did it occur, its peculiar 

 group of fossils would at once make it recognisable. Judging, however, from 

 the fact that these upper beds thin out as they extend westward from the 

 typical Moffat area, and that the zone of R. maximus seems to be absent from 

 Wigtonshire, it is not likely that it will be detected here. 



The grits and slates already referred to, which immediately overlie these 

 black shales, are similar in lithological character to those of South Scotland. 

 There they are the principal rock which forms the mountainous region known 

 as the Southern Uplands ; in County Down, however, though equally de- 

 veloped, their elevation is inconsiderable. As we pass southwards along the 

 promontary of the Ards, these rocks assume in places a more flaggy and 

 slaty character, and are in the neighbourhood of Greyabbey extensively quar- 

 ried and formed into roofing slates. On some of the surfaces exposed at these 

 quarries traces of organisms have been detected — tracks of annelids are abundant, 

 — but the only forms that admit of identification are Crossopodia Scotica, M'Coy ; 

 Nemertites tenuis, M'Coy; and Nemertites sp. 1 Still lurther south, near 

 Portaferry, bands of dark slate occur containing a group of Graptolites strik- 

 ingly different from any we have yet met with in the district. The palseonto- 

 logical break between these and even the highest of the Coalpit Bay zones is 

 almost complete. An examination shows that of the 14 species obtained from 

 them, only 3 are common to the two localities, and the II which make their 

 appearance for the first time indicate a much higher horizon for the containing 

 beds than those in the northern portion of the county. These latter beds have 

 no exact paleeontological representatives in the typical districts of Scotland or 

 Wales ; their nearest is, perhaps, the rocks of Upper Middle Silurian age known 

 as the Gala Group, occurring in the neighbourhood of Galashiels (1). The 

 presence, however, of Monograptus Riccartonensis and other Upper Silurian 

 species, and the absence of many of the Upper Moffat forms which occur in 

 that series in South Scotland, points to a higher place in the Silurian System 

 than that to which the Gala Group has been assigned. They are most prob- 

 ably the exact equivalents of the Hawick rocks of Scotland, which intervene 

 between the Gala beds and the Upper Silurian of Riccarton (2); but as no 



(1). Lapworth, on the Lower Silurian Rocks in the neighbourhood of Galashiels. —Trans. 

 Edinburgh Geol. Soc, Vol. II., p. 46- 



(2). Lapworth, On Scottish Monograptidse.— Geo. Mag., 1876, p. 550. 



