152 EDINBURGH GEOLOGICAL SOCIETY.

V.—The Irish or Western Portion of Dr A. Geikie's " Lake Caledonia Basin" of The Lower Old Red Sandstone. By G. HENRY EJNAHAN, M.B.I.A., &c. (Read 15th December 1881.) In the Preface to my " Geology of Ireland" (1878), I pointed out that the Lower Old Eed Sandstone of Ulster and north-east Connaught belonged to the Silurian formation. Subsequently I brought this subject more prominently forward before the Eoyal Geological Society of Ireland, and in the pages of the Geological Magazine. About this time Dr Geikie was preparing his paper " On the Old Bed Sandstone of Western Europe/' from which we learn that, in reference to the Scotch rocks, the writer had arrived at conclusions very similar to those to which I was led by the corresponding formations in Ireland. By the aid of a grant from the Eoyal Irish Academy, I and others were enabled to examine the Curlew and Fintona Moun­ tains more carefully, and an epitome of our Eeport on those rocks appears in their Proceedings. In this paper I propose to lay before the Society a short description of the rocks in the whole of the western continuation of Dr Geikie's Caledonia Basin. The rocks of this basin, when followed into Ireland, appear at once at the north-east, on the coast of Antrim, and extend in a south-westerly direction across the island; the strata being much thicker and better developed where they disappear to the south-west under the Atlantic, than elsewhere. From the sections exhibited it will be seen that the same principle has been followed as that adopted by Dr Geikie in illustrating the changes in the strata of the Scotch basins. The Irish Silurians are of two types, viz., the " Lower Old Eed Sandstone type," or rocks of red colour, for the most part arenaceous (Red arenaceous series), but in some places argil­ laceous ; and the " Silurian type," or green rocks, generally argillaceous (Green argillaceous series). In the latter, calcareous beds or limestone often occur, and the fossils are always of marine animal life; while in the former the fossils are princi­ pally plants or fucoids. These divisions are purely lithological, not chronological; the rocks of the red types being sometimes above, and sometimes below those of the green types ; probably, however, if continuous sections could everywhere be seen, it would be found that the lowest and highest rocks of all belong to the " red types," that is to say, the littoral rocks first de- LOWEE OLD BED SANDSTONE. 153 posited and those last deposited are now red, while the deep water accumulations are principally of grey and green colours; these are they which contain beds of limestone. A peculiar feature of these Irish Silurians, to which special attention should be directed, is a zone yielding fossils of Caradoc-Bala types. This peculiarity does not appear to be confined to the strata in the western extension of the "Caledonia Basin," as similar fossils are also found in some of the rocks in Kerry, classed by Jukes as Silurians (western continuation of Geikie's "Welsh Lake Basin "). In reference to these Kerry rocks, however, it should be mentioned, that Griffith classed them with the Cambro- Silurians; while Mr M'Henry, of the Irish Geological Survey, who has lately visited them, inclines to the same opinion. Silurian Basin principally of the Lower Old Bed Sandstone type. In the north-east of Antrim there is an exposure of metamorphic rocks probably of Cambrian age. Lying on these rocks to the northward, in the neighbourhood of Ballycastle, are Carboniferous rocks of the Calp type (Lower Coal Measure of Scotland); while to the south-east, at Cushendun, are con­ glomerates and sandstones, which are possibly the littoral beds of the north margin of the Silurian basin. Westward and south­ ward the continuation of these rocks is obscured by the newer Cainozoic rocks; while still further westward they must be heaved southward by the faults of the Lough Neagh Valley. West of the Lough Neagh Valley, to the north-west of Money- more, in the county of Londonderry, there are red arenaceous rocks of uncertain age; these may possibly be an outlying por­ tion belonging to this basin. Farther southward, however, in the counties of Tyrone and Fermanagh, the undoubted Lower Old Red Sandstones appear from under the Carboniferous rocks in the tract which may be called the Fintona district, extending from Pomeroy westward to the valley of Lough Erne. These rocks, to the north, lie on metamorphic rocks, probably of Cambrian age; and to the east, at Pomeroy, and south-west, at Lisbellaw, on rocks which, from their fossils, should be classed as Cambro-Silurians. Further westward the rocks of the basin are heaved south­ ward by the faults of the Lough Erne and nearly parallel valleys, and next appear from beneath the Carboniferous rocks a little south-east of Lough Allen, in the neighbourhood of Drum- shambo, County Leitrim. The Drumshambo Silurians are cut off on. their western side by a fault which brings down the Carboniferous rocks against them; but they appear again almost immediately, and form a ridge of high ground (Counties Roscommon, Sligo, and North- East Mayo) to and beyond Ballaghaderreen (Curlew Mountain district). The rocks, however, are more or less shifted by 154 EDINBURGH GEOLOGICAL SOCIETY. faults; such as those on the lines of which are situated Loughs Key and Gara. In this tract, as far west as Ballaghaderreen, the basal rocks are not exposed, but to the westward of this, and southward of Charlestown, they appear lying on metamorphic rocks of uncertain age; because, as shown in a paper read by me before the Eoyal Irish Academy, they may be either Cam­ brians or Cambro-Silurians, though probably the latter * Farther westward the rocks of the basin appear to have been denuded away, as they do not occur again until we reach the Croaghmoyle district, near the head of Clew Bay, in Mayo, where we find littoral rocks lying on apparently metamorphosed Cambro-Silurians. From this, apparently, the Silurian Tocks continue out westward along Clew Bay to Clare Island, under the Carboniferous rocks, as they are to be seen at Beltra, Cur- raun, Clare Island, and Louisburgh. Further southward, from Lough Mask to the Atlantic (Lough Corrib, Toormakeady, Formnamore, Mweelrea, and Culfin dis­ tricts), is a detached, nearly parallel, portion of the basin. This detached portion, although it was evidently deposited in a dis­ tinct trough from that in which the rocks to the north accumu­ lated, was probably a portion of the main basin, which is now disconnected by denudation, as the Croaghmoyle rocks are so similar to those of Toormakeady. The rocks, however, in the south, now detached, portion axe peculiar, inasmuch as they suddenly and rapidly change their character and increase in thickness as they are followed westward. To the westward, between them and the rocks of Clew Bay, there appears to have been a deep trough, in which rocks of a type different from both were deposited. After this general description, the following details of the rocks may be given, beginning with those of the County Antrim. The Cushendun rocks are conglomerates, or conglomeritic, and seem to have been deposited on the margin of the basin. Very little more can be said about them, as they are so spar­ ingly exposed. The rocks north-west of Moneymore, County Londonderry, are pebbly red sandstones, which seem to be connected with the Lower Old Eed Sandstone of the Fintona district by small outlying patches. Some of these small patches do not appear on the published maps. The rocks of the Fintona district are, on the eastern side,

* Griffith, Jukes, and the other older observers mapped these rocks as old metamorphic; while in the new map they are classed with the overlying unmetamorphosed Silurian. This latter classification is undoubtedly incorrect, as they were metamorphosed, upturned, and extensively denuded prior to the deposition of the Silurians upon them. LOWER OLD RED SANDSTONE. 15& nearly all of a red colour, and] arenaceous, having in them, at about 1800 feet above their base, interstratified eurites and tuffs. But near Six Mile Cross .there,appear to be at their base bedded eurites and limestones, over which are red conglomeritic rocks, which are succeeded by green rocks with subordinate limestone; and over them apparently there are other red rocks. This sec­ tion is very similar to that near Ballaghaderreen, to be men­ tioned presently, except that in the green rocks and limestones of the latter fossils have been found. It was from near Six Mile Cross that Griffith was sent the fossils, the age of which he could not decide on, and, curiously enough, I am now in a similar predicament. I have been sent fossils from this place by Mr Thomas Plunkett, M.R.I.A., of Enniskillen, which occur in & boulder of a green argillaceous rock very like many of the rocks in situ in the immediate neighbourhood, but also very like some of the Pomeroy rocks 6 miles to the eastward. The principal fossil, Leptoena seriea, is a Caradoc-Bala form, and very common near Pomeroy, but, at the same time, it is the most common fossil in the Toormakeady Silurians, and also in the Mweelrea Silurians ; still it is safer to regard the block as a boulder from the Pomeroy Cambro-Silurians until fossils are found in the rocks in situ of the Six Mile Cross section. Towards the west of this area the rocks are almost entirely red and arenaceous, a peculiarity being the inlying bits of shale; these we shall have to refer to again. In the south-west of this area, at lisbellaw, there is a remarkable sudden local accumula­ tion of massive conglomerates, the inlying fragments being im­ portant, as they are from "baked rocks " (hornstone principally), which occur nowhere nearer than the hills of Leitrim (Manor Hamilton). That these conglomerates are only a local mass is proved by their being absent in the section to the north-east, at the north-east end of Lough Eyes, and to the north-west, in the railway cutting. As hereabouts there are whinstones protruding into the Cambro-Silurians, it may be suggested that the frag­ ments were derived from a mass of baked Cambro-Silurian in the vicinity, which is now covered up by the Carboniferous rocks. At the Drumshambo exposure, which is very small and un­ satisfactory, there are rocks of both the red and the green types ; the exposure apparently showing the rocks just above and below the junction of the two kinds. In the eastern portion of the Curlew mountain range there are only exposures of the upper red beds capped in places by outlying patches of tuff; in the low ground to the west, however, between Ballaghaderreen and Charlestown, lower beds make their appearance. The section here is most instructive :— 1. Metamorphic Cambro-Silurians or Cambrians. 156 EDINBURGH GEOLOGICAL SOCIETY.

2. Unconformability. 3. Eed and purplish conglomerates, sand­ stones, and sandy shales, . . . 1,500 feet 4. Green conglomeritic sandstones and shales, with their impure fossiliferous limestones, and a few red shales, . about 4,000 feet 5. Purplish, red, and greenish conglomeritic sandstones and sandy shales, . . about 5,000 feet 6. Eurites, tuffs, and limestone (?), . . over 200 feet

10,700 feet The rocks in groups 4 and 5 are very similar, except as to their colour, and that in No. 4 there are some fine argillaceous rocks and impure limestones. The arenaceous rocks in the three groups, 3, 4, and 5, are made up of pieces of the underlying metamorphic rocks; a most important consideration, as recently' the latter have been classed as one group with the rocks of No. 4. As before pointed out, this section is very similar to that near Six Mile Cross, in the Fintona district, having red rocks above and below the green. A little above the base of group 4, in the Cashelduff stream, there are, in impure limestones, fossils, principally of Upper Llandovery types, although two or three are of Caradoc-Bala types; but to the eastward, in Griffith's fossil locality, at the mearing of Glenmullynamaher and Uggool, in a very similar limestone, they are principally of Wenlock types; although in shales below and above them, in Glenmullynamaher, to the north, and Cloonnamna, to the south, the fossils are of Upper Llandovery species. Here therefore, as elsewhere in Ireland, there is a mixing up of fossils which in Wales are characteristic of distinct groups of rock. The rocks in groups 3 and 5 are lithologically similar, and could not be separated if No. 4 had not intervened. Furthermore, the rocks of No. 4 are very similar in composition to those above and below them, the marked difference being as to the colours and the fact that those of the light colours alone carry fossils. The coarse sandstones in each of the three groups are made up of the same detritus, and contain similar thin shale inliers ; in fact, it would seem as if No. 4 ought not to be recognised as a separate group, but as an accidental inlier in the rocks of the Eed arenaceous series. No satisfactory section of the rocks of group 6 is exposed, although enough of them is seen to show that they lie in the trough of a synelinal curve; also that subordinate limestones are associated with them. In the country west of Lough Key a thin impure limestone was observed in situ; while in the district north-west of Ballaghaderreen fragments of limestones like those of Eskerboy, County Tyrone, are frequent. LOWER OLD RED SANDSTONE. 157

The sections in the Curlew mountain district are so far instructive, that from them we learn that the rocks of the " Lower Old Eed Sandstone type " (Red arenaceous series) were accumulating at the margin of the Silurian sea, while more argillaceous and calcareous rocks were being deposited in deeper or quieter waters in which lived and died inollusca characteristic of the "Welsh Silurian sea; also that subsequently these latter rocks were covered by accumulations of the " Eed arenaceous series." In two places in this area, to the north of Doon, on the west margin of Lough Key, and at Moygara, to the north-west of Lough Gara, tracks (doubtless Crustacean) similar to those found in the Glengariff Grits, Valencia, County Kerry, were observed. Westward of the area last described there must have been a hill of old rocks in the Silurian sea, because farther westward at Croaghmoyle the rocks are littoral, being massive conglomerates or breccia, except in one locality to the westward, where apparently under the conglomerates is a considerable thickness of red sandstone and sandy shales; there are also in places lime­ stones at the base. Still further westward in the vicinity of Clew Bay, near Molrany, in Curraun, in Clare Island, and at Louisburgh, there are also conglomerates and red sandstones; but associated with them are greater or less thicknesses of red shales and slates. Northward of Louisburgh the argillaceous rocks are of con­ siderable thickness; while in the hill to the westward are in- terstratifications of shales and sandstones, with subordinate conglomerates. The Louisburgh beds are perhaps the newest accumulations in the whole of this portion of the basin; they approach more nearly in lithological character to those of the Glengariff Grits than any of the others. The relations between the rocks just described and those in the detached portion, or nearly parallel trough, extending from Loughs Mask and Corrib to the Atlantic, at the Culfin, have next to be mentioned. To the west of the north portion of Lough Mask, that is, in the Slieve Partly or Toormakeady district, there are rocks nearly identical with those of Croagh­ moyle, except that in limestones at the base of the group are fossils of Caradoc-Bala types, one very characteristic, Leptoena serica9 being the fossil mentioned as found in a loose block near Six Mile Cross, County Tyrone. These rocks westward in the Formnamore Mountains rapidly lose their character and change into green sandstones, grits and shales; while still further west, in the Mweelrea Mountains, they are partly of the Toormakeady type and partly of the Formnamore type ; while a little above their base is a fossiliferous zone, carrying fossils similar to those of the Toormakeady rocks (Caradoc-Bala types). 158 EDINBURGH GEOLOGICAL SOCIETY.

North of the Mweelrea Mountains, and south of the Louis- burgh beds, from which they are separated by a fault with a downthrow to the northward, is a small independent trough of Silurian rocks. These appear to have belonged to the green argillaceous series, but now are all metamorphosed, except a small area at Creggaunbaun (to the south-west), where the rocks are very fossiliferous, the fossils being of Wenlock and Lower Llandovery forms, similar to those at Kilbride (Lough Mask). Returning again to Lough Mask. The rocks of the Toorma­ keady, the Formnamore, and Mweelrea types are bounded on the south by a fault, with a downthrow to the southward extending from Lough Mask to the Little Killary, on the Atlantic, and south of this fault the rocks are of new types. To the eastward at Kilbride (Lough Mask), and adjoining Lough Corrib, the rocks are similar to those of Creggaunbaun, and contain similar fossils. Farther westward, as we approach the Maum Valley, we have red rocks above and below; the first (SalrocJc slates) containing, as their characteristic fossil, Lingula Syrnonddi, pronounced by Davidson to be an Upper Llandovery form. In the lower red beds no fossils have been found; while in the intermediate green and grey beds all the fossils are of Lower Llandovery and Wenlock forms, the first predominating. Still further west there are, below, the Gowlaun group; in the centre, the Lough Muck group; and above, the Salrock slates. The first to the westward, or adjoining the sea, is nearly alto­ gether slates, which change eastward first into green grits, and near Lough Fee and the Blackwater into red conglomerates with subordinate shales. The fossils in these are all of Lower Llan­ dovery forms. In the rocks of the Lough Muck series the fossils are of Lower Llandovery and Wenlock forms, except on one zone, where they are of Caradoc-Bala types. The fossils of the Salrock slates have already been mentioned. These rocks are lithologically similar to the slates at Louisburgh, but in the latter no distinct fossils have been found. This portion of the basin between Loughs Mask and Corrib, on the east, and the Atlantic, on the west, is very instructive, on account of the rapid changes both lithologically and as to the fossils, not only from east to west, but also in places across that direction ; also in the changes between its rocks and those in the main basin to the north. To the eastward, the Toorma­ keady rocks and those of Croaghmoyle are identical; but to the westward there are great changes, one set of rocks being found south of the Killary, another in Mweelrea, a third in Creggaunbaun, and a fourth at Louisburgh; as shown in the accompanying section. In connection with the sedimentary rocks of this basin there are eruptive rocks, principally purplish and greenish eurites, or LOWER OLD RED SANDSTONE. 159 basic felstones, similar to the rocks found in the Scottish portion of the " Lake Caledonia basin." These eurites are best developed towards the west, where, in the south trough, they occur in a massive bed or beds at the junction of the Gowlaun and Lough Muck groups; that is, about 2700 feet above the base of the Silurians. These can be traced eastward as far as the Maum Valley. There is also a higher bed in the Salrock slates. At or near the base of the rocks of the Mweelrea, Formnamore, and Toormakeady district there are together from one to seven beds there in places in mass, as in the Formnamore district between Loughs Nafooey and Mask. These rocks are continuous from the Atlantic to Lough Mask under the rocks of the different types, and probably are on the same geological horizon as the eurites between the Gowlaun and Lough Muck groups. In the Silurians round Clew Bay, as also those near Beltra and Croaghmoyle, there are not any eurites recorded; but at the base of the latter there are limestones similar to those asso­ ciated with the Toormakeady eurites. In the westward part of the Curlew Mountain district, near Ballaghaderreen, there are elvans and other root rocks; while east of Lough Gara the highest rocks exposed are thick massive tuffs or tufifose rocks, each mass being made up of a series of beds from 1 inch to 4 or 5 feet thick. This would seem to indicate that the flows of eurite graduated eastward into beds of tuff and tuffose rocks. In the Fintona district there is at, or outside, the northern limits of the Silurian rocks an exposure of the granite or elvan roots of the eurites. The eurites seem to occur on two horizons. Those on the upper are very similar to those of the Toorma­ keady district, being made up of beds of eurite, tuff, and lime­ stone ; except for one peculiarity, which is that all the bedded portions of the eurite are affected by a structure that splits them up into slates and flags.