Swanston— Silurian Rocks of Co. Down. 
hi 
3. Massive black slates and shales rich in fossils ; the following are a few 
of the most characteristic : — 
Rastrites peregrinus, Barr. 
Monograptus gregarius, Lapw. 
,, concinnus, Lapw. 
,, Sandersoni, Lapw. 
„ triangulatus, Harkn. 
,, spiralis, var. fimbriatus, 
Nich. 
Diplograptus folium, His. 
,, TAMARISCUS, Nich. 
,, VESICULOSUS, Nich. 
Dimorphograptus elongatus, 
Lapw. 
Discinocaris Browniana, Woodw. 
Dawsonia campanulata, Nich. 
This area is considerably covered with shingle and sand, and the rocks are 
intersected by several faults carrying them to the north-east ; their thickness 
may be set down at about - - - - - 100 feet. 
4. A dyke composed of pale grey finely crystalline calcareous rock 
running easterly (1) .... 6 feet. 
5. Pale grey and greenish mudstones, greatly shattered and unfossili- 
ferous. ...-- about 65 feet. 
Near the base of these mudstones is a thin band of black shale greatly 
crushed, in which no fossils have yet been detected, but higher up in the mud- 
stones a fragment of black shale, about two feet in diameter and several inches 
thick was found containing well-marked specimens of Dicellograptus Forcham - 
mcriy Geinitz. 
6. Black shales highly indurated and containing numerous flinty bands, 
and nodules and veins of iron pyrites. The less altered portions are 
rich in fossils and have yielded the following : — 
Climacograptus tricornis, Carr. 
„ ccelatus, Lapw. 
„ bicornis, var. peltifer, 
Lapw . 
Dicellograptus elegans, Carr. 
Dicranograptus formosus, Hopk. 
Dicranograptus zic zac ,var. min- 
nimus, Lapw. 
Acrotreta Nicholsoni, Dav. 
Acrothele granulata, Linrs. 
These shales are too much shattered and crumpled to allow of their thick- 
ness being exactly estimated. It may, however, be roughly put down at about 
50 feet. They form the ridge of a sharp anticlinal, which has for its axis 
(1). Geological Survey of Ireland — Explanatory memoir to sheets 37, 38, & 39. p. 31. 
