1849.] NICOL ON THE SOUTH-EAST OF SCOTLAND. 57 



flags are also the most closely allied in mineral character to the Silu- 

 rian formations of the region now considered. The coarse white sili- 

 ceous grits common among the Cars doc sandstones aie unknown, so 

 far as I have observed, among the old rocks in the south-east of Scot- 

 land. The only beds that approximate to these grits in mineral 

 aspect are some in the south of Roxburghshire, and thus in what 

 is probably a newer part of the series (perbaps equivalents of some 

 part of the upper Ludlow rocks?). The most important pecu- 

 liarity of the Welsh rocks, is the great abundance of organic life 

 which they exhibit when compared with the few traces found in the 

 north. They seem also to differ somewhat in mineral composition, 

 containing a much greater amount of felspathous and apparently vol- 

 canic materials than are seen in the Scottish strata. Even mineral 

 and metallic products are, with a few exceptions, rarer in the latter 

 deposits ; — probably consequences or at least indications of a less fre- 

 quent contemporaneous igneous action. It is an interesting question 

 how far we can connect the more or less abundance of life in these 

 ancient seas with the variety of mineral ingredients thrown into their 

 waters by volcanos existing at the time*. 



The Silurian beds disco veered by Colonel Portlock in Tyrone, are, 

 however, the nearest equivalents of those nov/ described. The mineral 

 characters of the rocks do not appear very different, and in both coun- 

 tries they are characterized by the comparative scarcity of calcareous 

 matter. The Irish strata also lie in the direct continuation of the 

 northern part of the Scottish mountains, from the termination of which, 

 near Portpatrick, they are only separated by an interval of 100 miles. 

 From Peebles-shire the distance is nearly 200 miles, and the agreement 

 in organic remains is perhaps closer than might have been expected, 

 more especially as not more than three of the Peebles-shire fossils appear 

 among those collected by Mi\ ]\Ioore in an intermediate locality. It 

 is also worthy of notice, that whilst the Grieston graptolites have only 

 serratures on one side of the axis, those from Wigtonshire generally 

 show these on both sides : hence the latter probably belong to another 

 part of the series, and perhaps correspond with the patch of Silurian 

 schists discovered in Fermanagh in Ireland, which contain the G, 

 pristis and other foliaceous species like those in Wigton. These 

 strata lie further south than the Tyrone beds with which the Peebles- 

 shire deposits have most affinity, and Colonel Portlock f also places 

 them in a higher part of the series. In both countries therefore, the 

 older rocks occur on the north, the newer on the south, a coincidence 

 confirming the classification of the Scottish deposits here proposed. 

 In another point, the geological history of the two districts also cor- 



* The contemporaneous traps of the English Silurian deposits are described by 

 Sir R. I. Murchison in his great work on this formation. See Silurian System, 

 pp. 75, 269, 401, &c. ; comp. Journ. Geol. Soc. vol. iii. p. 171-175. In this place 

 the author ascribes the comparative rarity of animal life in certain portions of the 

 English strata, to the intense igneous action which accompanied their deposition. 

 The opposition to the view in the text is only apparent, as the eruptions which 

 in their immediate vicinity and during the time of their most violent action were 

 suflScient to destroy life, might yet be favourable to its development during the in- 

 tervals of repose and at greater distances, by introducing more calcareous and 

 other substances into the water. f Report on Lond. p. 232. 



