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 found 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. 
