Ii6 
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- 
lites, 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. 
