n6 Swanston— Silurian Rocks of Co. Down. 



more or less parallel, and often miles in length. They are found at intervals 

 throughout the northern half of the uplands, from the North Sea to the Irish 

 Channel, and follow the general direction of the strike of the beds. Their 

 well-marked mineralogical character and their peculiar fauna at once distin- 

 guish them from the monotonous greywackes, and afford the geologist data 

 upon which to work out the interrelation of the rocks of the entire series. 

 Without attempting to follow the author in his description of the many sections 

 and diagrams given in elucidation of his subject, I may briefly state that the 

 rocks are found to be arranged in elongated anticlinals, running in a direction 

 from about N.E. to S.W., and that the sub-parallel bands of black graptolitic 

 shales of the Moffat Series form the axes of these anticlinals, and are only seen 

 where denudation has cut sufficiently deep to expose them. The Moffat Series 

 is thus proved to have been one continuous deposit, inferior in position to the 

 prevailing greywackes through which it rises. Where best exposed the Moffat 

 Series naturally falls into three divisions - an Upper, Middle, and Lower — 

 named respectively the Birkhill, Hartfell, and Glenkiln Shales, from the several 

 localities in which each division attains its greatest development. These three 

 divisions possess distinctive lithological features ; but as the entire group is full 

 of perplexing contortions and inversions, and as the continuity of the beds is 

 greatly broken by numerous faults running in different directions, it was impos- 

 sible to work out their sequence from stratigraphical evidence alone. The aid 

 afforded by the contained fossils is, however, most satisfactory, and has enabled 

 the author not only to prove that the divisions must be assigned to different 

 geological periods, but they are each divisible into several distinct zoological zones. 

 The correlation of these various sub-divisions with the typical Silurian area 

 in Wales, and their foreign equivalents, is gone into with great minuteness by 

 the author. I shall content myself by merely giving the conclusions arrived at, 

 and beg to direct those anxious for further details in this department of the 

 subject to the paper itself. 



The lowest, or Glenkiln Shales, have been referred to the Upper Llandeilo, 

 their nearest representatives in Wales being beds of that age yielding Grapto- 

 Jites, in the neighbourhood of Llandridod, Meadow Town, and Aberiddy Bay. 

 Their American and Swedish equivalents bear out this view, and point to a 

 high position for them in the Upper Llandeilo Series. The Hartfell beds have 

 been proved to be the attenuated representatives of the Bala and Caradoc, and 

 the Birkhill Shales correspond with the Coniston mudstones of the Lake Dis- 

 trict, which are of Lower Llandovery age. The accompanying table will 

 give more clearly these Scottish divisions and sub-divisions, and I shall now 

 proceed to give the evidence that has enabled me to append the columns relat- 

 ing to County Down. 



