Swanston— Silurian Rocks of Co. Down. 
119 
fibratus , Lapw. ; Climacograptus tubuliferus , Lapw. ; and Dicellograptus For- 
chammeri , Geinitz. The remaining four species are common to the Hartfell, 
and the underlying Glenkiln Shales. So far as can be judged from the meagre 
list of fossils obtained at Orlock Point, the beds there exposed also belong to 
this group. 
Again returning to the section in Coalpit Bay, we find the mudstones 
which have just been referred to the Upper Hartfell Series are separated from 
the next beds in ascending order, by a dyke which runs almost parallel to the 
strike, and which seems to occupy the line of an east and west fault. The 
rocks immediately south of this dyke are very rich in fossils, 36 species having 
been obtained from them, although the area exposed is of but very limited 
extent. The palaeontological break between these and the preceding beds is 
most complete, not one of the extensive series of fossils being common to the 
two rock groups. Not only are the species different, but a marked change 
also takes place in the generic character of the fauna. All the old types of 
compound forms such as Ccenograptus, Thamnograptus, Dicellograptus, 
Dicranograptus, Leptograptus, &c., have disappeared, and their places have 
been supplied by the simpler genera Monograptus and Rastrites. 
This extensive series of fossils admits at once of the correlation of the con- 
taining beds with the Birkhill Shales of the Moffat District, with the lower 
division of which as exposed at Dobbs Linn they almost exactly correspond, 
both in lithological character and fossil contents. In these beds also for the 
first time in the County Down rocks are we enabled to identify the life-zones 
so ably worked out in the Scottish beds. The zone of Diplograptus aciim- 
minatus , which is at the base of Birkhill Shales, is found, in part at least, quite 
close to the dyke above mentioned. The well-marked zone of Diplograptus 
vesiculosus follows it with a rich assemblage of species, often beautifully pre- 
served. The succeeding zone of Monograptus gregarius is also easily recog- 
nised by the immense profusion in which that fossil, associated with Rastrites 
peregrinus (type) and other forms abound in it. 
A series of greenish grey mudstones, with thin layers of white clay, sepa- 
rates the foregoing zones from the next fossil bands. The upper horizon of this 
barren group is in Scotland taken as the boundary line between the Lower and 
Upper Birkhills. The only other zone which can with certainty be recognised 
at Coalpit Bay, is that of Diplograptus (Cephalograptus) cometa, which occurs 
at the top of the fossil bearing beds. It is just possible that the zone of Mono- 
graptus spinigerus—Sedgwicki, which follows the latter in Scotland, may be 
(1). Quartl. Journal Geol. Soc., London. Vol. XXXIV., p. 316. 
