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No. 26. 



EVIDENCE OF DISPLACEMENT OP CARBONIFEROUS STRATA, 



COUNTY SLIGO. 



By ARTHUR E. CL^IRK, B.A., 



Trinity College, Dublin. 



(communicated by MR. L. B. SMYTH.) 

 (Read Apeil 24. Printed July 6, 1923.) 



This paper is the result of a detailed survey of the district represented by 

 Sheet 12, Co. Sligo, 6" Ordnance Sui-vey. It includes the village of Dromore 

 West, on the western side, and Aughris on the east, including pai"t of the 

 northern coast of Co. Sligo, with a few miles of country inland. 



The formation is lower Carboniferous, consisting of anticlinal strata of the 

 Mountain Limestone Series. To the west the beds consist of shales, in some places 

 dark, almost black, and very soft ; in other places ochreous, harder, occasionally 

 with harder and softer beds alternating. 



To the east of these beds of shales there is an anticline of calcareous sand- 

 stones underlying the shales on the west, and alternating with dark micaceous 

 flagstones. The boundary line between the sandstone series and the dark 

 shales to the west is incorrectly marked on the Geological Survey maps. The 

 N.-S. fault passing through Carrickpatrick is represented as passing through 

 the shales— c?"' — instead of passing through the highest beds of the sandstone 

 anticline — cZ\ The true boundarj"- line is marked on the map (fig. 1). A bed 

 of oolite near the junction of shales and sandstone, which, according to the 

 memoir, lies at the base of the shales and overlies the sandstone, really lies 

 between two beds of sandstone. 



The sandstone anticline is visible for about two miles; then the beds dip 

 beneath greyish compact limestones, which rest conformably on the sandstone. 



In addition to the sedimentary rocks described above, there are a number of 

 greenish microcrystalline dolerite dj'kes, and the distribution and structure 

 of these dykes form the basis of this research. Some of the dykes of the district 

 are marked on the Geological Survey maps, but their positions are in some cases 

 incorrect. 



In this district, on the western side, the dykes ai^e first seen on the 

 Carrownrush shore at PoUbreaii. They cross a small bay, and pass through 

 land for a short distance; they then emerge at Oughmore, and proceed out to 

 sea in the direction of Aughris Head, towards a point considerably north of 

 Kilrusheighter (see map). 



The Geolc^'ical Survey mark only two dykes crossing the small bay at 

 PoUbrean, and three going out to sea on the east. They represent the two dykes 

 as converging, and the third as a short line to one side (see fig. 2, A). 



At PoUbrean, however, there are really three dykes — ^the two marked, and a 

 third, of a different type, between them, passing into an inlet in the land. It 



SOIENT, PROO. K.D.S., VOL. XVH, NO. 26, 2 t 



