494 Proceedings of the Royal Insh Academy. 



In continuation of the accoimt of tlie " Lower Old Eed Sandstone" 

 basin on the -westward, short descriptions of the Silurians of S. "W. 

 ilayo, jST.^. Gralway, and Keriy, appeared in the original Eeport. In 

 the Croagh Moyle district, the "Lower Old Eed Sandstone" consists 

 principally of conglomerates, which lie unconformably on Canibro- 

 Silurians or Cambrians {Arenig rochs). In places, under the con- 

 glomerates, are red sandy shales and sandstones ; while very similar 

 rocks occur in the country to the jS^. W. jS'o eruptive rochs occur in 

 them ; but in places at their margin are limestones in the ujiderlying 

 rocks,^ which are somewhat similar to the limestones, the associates 

 of the eurites in the Fintona district, to the X. E., and in the Toor- 

 makeady district, a little to the southward. 



Adjoining Clew Bay, near ]y!ulrany, in Clare Island, and near 

 Louisburgh, the rocks are somewhat similar to those of Croagh ]\Ioyle, 

 except that red argillaceous rocks are more or less developed. In the 

 north portion of the Louisburgh district these argillaceous rocks occur 

 in mass ; while in the south portion they are rnterstratified with the 

 arenaceous. The Louisburgh beds constitute perhaps the latest divi- 

 sion of the "Lower Old Eed Sandstone" in Mayo, as they lie against 

 metamorphic rocks (having been brought into that position apparently 

 by a fault), an unaltered portion of which, at Creggaunbaun, contains 

 fossils principally of Wenlock tjrpes. The relative positions of the 

 rocks of Louisburgh, those of Creggaunbaun, and those further south 

 in Mweelr'ea, are shown in the horizontal section, Plate XYL, fig. 5. 



In the countiy to the south of the Croagh Moyle and Creggaun- 

 baun rocks, and separated from these by Cambro- Silurian rocks, is the 

 long narrow tract, extending fi'om Lough Mask by the north of Kil- 

 laiy Bay to the Atlantic, including the Toormaheady, Formnamore, and 

 Mweelrea districts. Here, on the eastward (Toormakeady), there are 

 massive conglomerates of the "Lower Old Eed Sandstone " type, 

 under which are fossiliferous limestones and shales. The conglome- 

 rates to the west of these in Formnamore merge into thin-bedded 

 purple and green grits and shales ; while still further westward, in 

 Mweelrea, the rocks of the two types are more or less intermingled. 

 That these dilierent rocks are on one geological horizon seems proved, 

 as below them are the continuous interbedded eurites and tuifs, which 

 to the eastward are associated with the fossiliferous Toormakeady 

 limestones and tuffs. The fossils of the latter are principally of 

 Caradoc-Bala types, and similar fossils occur in green shales above the 

 eurites to the westwai'd in the Mweelrea district. This narrow area 

 is remarkable ; as there are not only rocks of the " Lower Old Eed 

 Sandstone" type and those of the "grey and green" series on the 

 same geological horizon, but the fossUs therein are principally those 

 characteristic of the "Welsh Caradoc-Bala rocks (Cambro- Silurian). 



^ Symes has stated that these calcareous rocks may be of Carboniferous age, 

 being due to infiltoration. 



