Swanston— Silurian Rocks of Co. Down. 109 



The Geological Survey in its progress northward has almost completed 

 the mapping of this area. The maps of the County Down were published in 

 1871-72(1). The Silurians are either set down on them in general terms as 

 Lower Silurian, or areas indicated by letters but whose boundaries are often 

 undefined, are assigned to the Bala or Caradoc, and to the Llandeilo. In their 

 examinations of the rocks several fossil localities were discovered which had 

 hitherto been overlooked, and in the " Explanatory Memoirs " lists of species 

 were published from time to time (2). 



The want of precision throughout the foregoing notices is no doubt due to 

 the highly disturbed nature of the strata and comparatively uniform lithological 

 character of the rocks, making the determination of their stratigraphical rela- 

 tions a matter of very great difficulty. This difficulty was much enhanced by 

 the absence, until very recently, of fossil evidence sufficient to allow their 

 correlation with other deposits. 



LITHOLOGICAL CHARACTER OF THE ROCKS OF THE 

 DISTRICT. 



Lithologically the rocks consist for the greater part of grey and purple 

 grits — occasionally conglomeritic — pale grey or greenish slates and flags, and a 

 few widely separated bands of black shales. The latter, though forming but a 

 small portion of the entire rocks here exposed, constitute a group which will 

 claim most attention in the following brief notice. In them are tound all the 

 fossils which are enumerated in the accompanying table, and it is from a careful 

 study of their fossil contents that we are enabled to arrive at an estimate of 

 their geological position, and that of the barren grits and slates associated with 

 them. The usual dip of the rocks is to the south-east and south, at angles 

 varying from 30 to vertical. Their continuity is much disturbed by faults and 

 contortions, and their upturned edges, where exposed, bear evidence of having 

 suffered much from denundation. Almost the entire rocks of the area are 

 obscured by drift, and with the exception of the coast-line, few good sections 

 can be seen. The grey and purple grits have not as yet yielded any fossil 

 remains, and it is only within the past few months that they have been 

 detected in the grey slates, a notice of which will be given further on. 



(1). Geological Survey of Ireland, maps 29, 36, 37, 38, 47, 48 and 49. 



(2). Explanatory Memoirs, accompanying maps 37, 38, 29, 48, 49, 50, and 61. 



