Swanston— Silurian Rocks of Co. Down. 115 



It will be noticed from the foregoing that a large proportion of the fossils 

 belongs to one class— namely, the Rhabdophora or Graptolithina. It is not my 

 intention to offer any suggestions as to the conditions which prevailed during 

 the deposition of these fossiliferous black bands, conditions alike favourable to 

 this singular group of organisms, and apparently unsuitable to others. It may 

 be observed, however, that no group affords a better index to the palaeontolo- 

 gist in his endeavours to unravel the sequence of rocks of Silurian age than 

 does the Graptolithina. The number of known species is very large, and it has 

 been ascertained that the vertical range of most of them is very restricted — 

 many, indeed, being confined to zones of but a few inches in thickness. The 

 immense profusion also in which they frequently occur stamps each zone with a 

 character easily recognisable by those who study their form and character. 



The similarity of the County Down rocks to those in South Scotland was 

 conjectured, as before stated, so early as 1815. This conjecture appears to 

 have been founded more upon the general appearance of the rocks of the two 

 areas than upon any positive evidence that was then obtained from them 

 Subsequent writers did little but repeat these early opinions ; and I am unable 

 to find any evidence adduced tending to prove their identity. 



The true relationship of the Silurian strata of South Scotland, and their 

 exact position in the geological system, were long a matter of debate, and claimed 

 the attention of many of the best geologists of Britain. To Mr. Charles Lap- 

 worth, of St. Andrews, is, however, due the honour of having finally succeeded 

 in unravelling the many knotty problems which the rocks of that district pre- 

 sented. A brief resume of the conclusions arrived at by that gentleman, as 

 given in a most exhaustive paper read before the Geological Society of Lon- 

 don (1), may here be given, as I trust to be able to prove that they bear with 

 equal force upon the rocks of the area under consideration. 



The Silurians of South Scotland are best shown in the neighbourhood of 

 the town of Moffat, in Dumfries. The prevailing rocks consist of a vast series 

 of grey and purple greywackes and slates, with occasional bands of conglome- 

 rates, all dipping somewhat uniformly to the N. N. W. at high angles. Exposed 

 in some of the deeper valleys and stream courses is another set of rocks of a 

 totally different character, consisting of black carbonaceous shales and slates, 

 swarming with Graptolites; associated with these beds are pale green and grey 

 unfossiliferous mudstones. These black shales and their associated mudstones, 

 which have been aptly designated the Moffat Series, occupy lenticular or boat- 

 shaped areas in the mass of the barren greywackes, and form extended lines 



(1.) Quarterly Journal Geol. Soc, London. Vol. XXXIV., p. 240. 

 H 



