Downloaded from http://trn.lyellcollection.org/ at Indiana University Libraries on May 11, 2015 [ 149 ] XI.—Notices on the Geohgy of the North Coast of the Counties of Mayo and Sligo in Ireland. BY ARCHDEACON VERSCHOYLE, M.A. Communicated by RODERICK IMPEY MURCHISON, Esq., P.G.S. [Read November 7 & 21, 1832.] THE country I propose to describe, comprises the northern districts of Mayo and Sligo. In constructing the map*, I have used for the former county, the excellent map of Mr. Bald, some fishery charts by Nimmo, and some local surveys; and for Sligo, a corrected copy of Larkin's Map. In connecting the two counties, I have used triangles resulting from the operations of the Ordnance Survey, most liberally communicated by Captain Portlock, with the permission of Colonel Colby; and where doubts and deficiencies in the details occurred, observations were made with a three-inch sextant, or Kater's Compass. The drawings (pp. 158. 162. 165. 167.) are from sketches taken on the spot, with every determination to sacrifice picturesque effect to accuracy. The only notice which has been published respecting this part of Con- naught, is a short but very able and accurate sketch in Mr. Griffith's paper on its Bogs, attached to the 4th Report of the Commissioners. The Ox mountain ridge extends through the district from south-west to north-east. It is chiefly formed of mica slate, hornblende slate, gneiss, and quartz rock, the strata dipping rapidly to the south. At the base occurs a conglomerate, alternating with sandstone and slate clay; and overlaid by the carboniferous limestone, which extends westward from Sligo to the Nefin group. This group is the commencement of the primary tract, reaching northward and westward to the ocean. The section from Lough Talt to Downpatrick Head, displays the succession of strataf. The Ox Mountains have a mean height of 1300 feet. On the north-west, the summits present abrupt rocky peaks; but the ravines are not of great •Plate XI. f Plate XI. fig. 3. Downloaded from http://trn.lyellcollection.org/ at Indiana University Libraries on May 11, 2015 150 ARCHDEACON VERSCHOYLE on the Geology of the North Coast of depth, and rarely reach to the base of the chain, being gradually lost in the slope of the sides. On the south-east, the ascent is more gradual, less rocky, and streams are more numerous. The principal passes through the range, are from Colloony to Ballisadere; from Coolany to Baltra by Carrick-na-Shouock; from the barony of Liney to Skreen by Knock-a-Chree; from the same barony to Easky by Lough Eask; from Tubbercorry to Ballina by Lough Talt; and the Pontoon near Foxford. The first, at Colloony, is very picturesque from the magnificent falls of the river over the limestone at Colloony and Ballisadere, and the much shattered and broken mountain of mica schist, called the Union Rock, which rises through the Union Wood; as if a course had been opened for the river by some violent convulsion, the further effect of which has evidently excavated the Bay of Ballisadere, assisted by the subsidence or removal of the limestone. The rocks surrounding Glen Lough at the foot of Benbulben, indicate a similar subsidence. On the south side also of the Ox range, the soil and even the strata of limestone, have been removed, and have probably formed materials for those immense ridges of water-worn gravel, called eskers, which are found in every part of the country. Near Tubb&corry, the surface fur a considerable extent, is an almost naked limestone rock, like the shore of the barony of Tyreragh, presenting the same rounded cavities in the surface, as well as the same water-worn aspect, wanting only the sea-weed and the recent shells, to complete the identity. The drainage of this district is through the pass at Collodny, by the Owenmore, which, rising from the west side of Cash Corran, receives at the Union Wood the River Arrow, and be­ fore it reaches Colloony that from Coolany, whose source is near Knock-a-Chree. The steep and rocky escarpments of the passes through which these streams flow, with the debris scattered over the surface, mark the effects of a powerful torrent. At the falls of the river, at Colloony, the limestone ceases, and thence to Ballisadere mica slate is laid bare, to a depth of fifty-five or sixty feet, lower than the limestone above the falls. The bank of the river at Coolany, about five Irish miles from the sea, is 298 feet above high-water mark, and the average descent is, therefore, 59*6 feet in the mile. The grey peak above the Union Wood and the precipices of mica slate which skirt the river below, are memorials of the ruin which has taken place; while the frag­ ments of the same rock so abundantly strewed in large boulders on the opposite side of the estu­ ary of Ballisadere, exhibit both the effect and the direction of the torrent, which evidently flowed from the south-west into Galway Bay, spreading thence to the north-east towards Foxford and Sligo, as indicated by the perpendicular faces and broken strata of the limestone mountains, pre­ sented to the south-west and west; by the caves in Cash Corran, perfectly resembling those into which the waters of the Atlantic rush along the coast of Erris; and especially by the direction of the ridges of limestone gravel throughout the baronies of Liney and Corran, and the southern part of Mayo. The second pass, Carrick-na-Shouock, derives its name from a bold and precipitous rock, which serves as a landmark to the harbour of Sligo, rising perpendicularly on three sides, and on the fourth with a very steep and difficult ascent. The road through the pass, winds over successive strata of quartz rock, gneiss, and mica slate, dipping to the south-east, and presenting to the north-west the broken edges of the beds. Proceeding westward, we next reach the pass of Carrow-na-Skea, which crosses the ridge below Knock-a-Chree, the highest summit of this chain (1714 feet), and on the north side descends Downloaded from http://trn.lyellcollection.org/ at Indiana University Libraries on May 11, 2015 the Counties of Mayo and Sligo in Ireland. 151 into Tyreragh, behind Skreen hill. On both sides, the foot of the mountain is covered by a stratified quartzose conglomerate, which extends as far as the pass of Lough Talt. The next communication between the baronies of Liney and Tyreragh, is through the mountain valley in which lies Lough Eask, the source of the river Eask. A small patch of granite borders the road which passes along the east shore of the Lough. The river Moy having received, before it reaches Foxford, an accession of water from the Guishaden, as well as the lakes Conn and Cullen (about forty feet above high-water mark), passes, below the town, through the mountain chain between Carrick o'Hara and Shraheen Hill. The course of this river from its source to the Atlantic below Ballina, is about thirty Irish miles; and the descent being 300 feet from its head to high-water mark, it has a fall of ten feet in the mile* or one-sixth of that of the Colloony river; and probably from this cause, the effects of the stream are less striking, the pass through the mountain chain being also less abrupt: the soil likewise has not been so entirely removed as at Colloony. Foxford stands on gneiss, from below which mica slate rises, at the hills of Shraheen and Bur- ran on the Moy. Lough Cullen communicates with this river near the town, having received the drainage of Lough Conn, by the rocky strait at the Pontoon, where the .waters have forced a passage through the beds of gneiss, leaving immense boulders scattered on the surface, even to the summit of the hills over the pass. It is remarkable, that throughout this district clay slate is never found either in situ or boulders. In the cliffs along the sea coast (see PI. XI.) the succession of strata is satisfactorily exposed. Commencing at the promontory of Tarmon in Erris, a district where primary rocks only occur, granite is quarried near Blacksod Point, and lies in beds which may indeed be called strata. The felspar is brick red, the quartz smoke grey and translucent, and the mica black: no imbedded minerals have been observed. In a small inlet on the shore, at the west end of the hill, below the Signal Tower, mica slate appears, dipping 15° to the east, and abuting against the face of the granite beds, which incline to the west 45°; and in a narrow cove, south of this inlet, the dip of the mica slate increases to 70°, its direction becoming south. Numerous veins of granite penetrate the slate, and in some instances contain fragments of it. Proceeding to the north-east, the coast presents only a succession of sand­ hills and beaches, except where a few reefs of gneiss or mica slate project into the sea. The first range of cliffs is at Annagh, the south headland of French- port, and consists of mica slate traversed by a trap dyke; but the north head­ land of this inlet is formed of gneiss, composed of red felspar and black mica. On approaching Scotchport, garnets appear, the mica increases, and the felspar gradually diminishes, till the gneiss becomes mica slate, which graduates into quartz rock, in the direction of Erris Head*. This formation constitutes the lofty and broken cliffs to Broad Haven, with the exception of a few places, * For additional observations on this part of the coast see p.