Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/133



We have therefore strong grounds for believing, without incurring the charge of generalizing too hastily, that the two mountain systems described, should be regarded as prolongations of the great northern and southern chains of Scotland, the former distinguished by the prevalence of mica slate, the latter by that of greywacké.

In the eastern part of Tyrone which intervenes between these two systems of mountainous ground, a coal formation occurs associated with that variety of limestone which is usually found underlying or alternating with the coal measures in Great Britain.

The position of this coal field offers a new analogy with Scotland, where the interval between the southern and northern mountains is principally occupied by a broad zone of rocks connected with the coal formation, upon which are placed those vast overlying masses of trap formation which constitute the Campsie hill, the Ochills, &c. and which, again, correspond both in their constitution and position, with the basaltic group which forms our third system. The assertion that the basaltic group of Ireland corresponds in constitution and position with that of the Campsie hills, Ochills, &c. requires perhaps to be stated more distinctly, and more in detail; and in the first place with regard to the correspondence of these districts in the internal constitution of the component rocks. From the description of the Ochills given by Mr. Mackenzie in the 2d volume of the Wernerian Transactions, it appears that the prevailing rock is a variety of lloeta trap, possessing characters intermediate between basalt and clinks tons, occasionally exhibiting columns, passing into greenstone, and in places associated with amygdaloid. Beds of claystone porphyry, of felspar porphyry and of compact felspar cap some of these eminences. The Campsie hills have been ably but more generally described by Col. Imrie in the same publication; be represents them as universally covered by a thick mass of flœtz trap, divided into beds, partly amorphous and partly columnar. The trap formation of the north of Ireland consists principally of basalt divided into beds, partly amorphous and partly columnar; the latter variety resembles clinkstone in the property whence that rock derives its name; the basalt passes into greenstone, as may be seen in the mountain of Slievemish and other places; and it alternates with amygdaloid. A formation of clay porphyry also occurs in the center of the basaltic area.

In the second place, with regard to the position of the Scotch and the Irish trap districts, there are points of agreement and points of differences the great features of agreement are those mentioned in the text, namely, that both occur in the vicinity of coal fields, and in a position intermediate between the northern and southern mountain chains. The points of difference are, that the Campsie hills seem, at least along their southern border, to repose immediately on the coal measures; and the same observation maybe applied to the junction of the Ochills with the coal held of Clackmananshire at Westertonn. Whereas the basalt of Ireland is generally separated from the coal measures by several intervening beds of considerable importance and of much more recent formation, which appear to be altogether wanting in Scotland. It may be even objected to the instance of Cross hill near Fairhead, which is cited towards the end of this introduction as an example of the occurrence of trap in Antrim under the same conditions with that of Scotland, namely, in contact with the coal measures, that the trap in this instance and through the whole range of Fairhead, assumes a character so widely different from the compact and small grained basalt of the neighbourhood, (being a highly crystalline greenstone) as almost to warrant the suspicion of its being a distinct formation, not withstanding its close geographical proximity to the great basaltic mass. I confess myself however to be fully persuaded that this suspicion will on further examination be found groundless. The rock in question closely agrees in its texture with that of Slievemish, which it is quite impossible to detach from the general mass of trap. The island of Arran appears to form an important link in the connection I have endeavoured to trace, between the opposite coasts of Ireland and Scotland; the center of this island is occupied by a primitive district, comprising granite, mica slate and syenite; the mica slate extends to and skirts the N.W. coast. The N.E. coast presents a small coal formation precisely analogous to that of Ballycastle, with beds of breccia and of red sandstone resembling those which occur in the Antrim coast near New own Glens; this sandstone forms the prevailing substratum through the whole south of the island, where it is covered by a singular columnar clay porphyry, and by greenstone, both of which rocks are generally referred to the flœtz trap series. It seems probable that the porphyry which occurs on the opposite coast of Cantire at Devar near Campbelltown, and the syenitic rock which forms the crag of Ailsa may be considered as connected with these formations. The whole island is traversed by numerous dykes of basalt and pitchstone. A similar structure is continued through the corresponding portions of Bute. And an exact resemblance prevails between the northern and southern districts of these islands and the opposite coasts of the estuary of the Clyde in which they are situated; thus the primitive rocks, forming the north of those islands, are resumed on the north of the estuary in Cowal and Cantire; while the sandstone, breccia, and trap rocks of their southern district are continued in that direction through the Cumbray isles, and at Largs and the whole southern bank of the Clyde: the line of junction between these secondary and primitive formations seems to pass near the southern extremity of the Peninsula of Cantire, where a small secondary district, containing, like that of Arran, some indications of coal occurs near Campbelltown. See Jameson Mineral of Scottish isles, vol. 1. p. 134.

Whether any connection can be traced between the flœtz trap of Ulster and the extensive deposits of similar rocks in the islands of Sky, Egg and Mull, is a question which will be examined with greater advantage when the geology of these islands has been further elucidated by the able descriptions of Dr. Mac Culloch. It should appear that in these islands strata of shell limestone containing belemnites and ammonites, and appearing to be of more recent formation than those associated with the coal fields of the main land, occur; hence we may perhaps be led to expect that they will be found to exhibit a still nearer approach to the structure of the hills which constitute our third system. Towards the west, the limestones and sandstones associated with the coal formation extend from the banks of Lough Neagh to those of Loch Corrib in Connaught, passing by Lough Erne and Lough Allen; but the overlying trap is confined to the district described in this paper, with one trifling exception which is presented by the Corliew mountains in Roscommon, two miles to the north of Boyle, where the sandstone is covered by a cap of greystone: the structure of these hills, therefore, agrees still more nearly with the trap ranges of Scotland than does that of the great Ulster group. Before