40 ' GEOLOGY. 



These sheets include parts of Roscommon, Mayo, and Sligo (Bal- 

 laghadereen, Kilkelly, and Kiltimagh, with the towns French-park 

 and Elphin). 



The ground is comparatively low, save in the north of Sheet 76, 

 where a ridge of Igneous and Silurian rocks rises to heights of 618 

 and 775 feet ; the lower grounds (chiefly Carboniferous rocks), at an 

 average height of 300 feet, are covered in places by extensive alluvial 

 flats and bogs. Five rivers drain the district — the Moy, Lung, Suck, 

 Dalgan, and Breedoge. The formations occurring are : — Aqueous : 

 Alluvium, Bog, Drift, Lower Carboniferous Limestone, Lower Carboni- 

 ferous Sandstone, Old Bed Sandstone, Upper Silurian (Wenlock Lime- 

 stone, "Wenlock Grits, Shales, Slates, and Flags, Upper Llandovery 

 Beds). Igneous : Basalt, Quartziferous Porphyry, Felstone, Melaphyre, 

 Felspathic Ash ; these are confined to the north of Sheet 76 and the 

 eastern corner of Sheet 77. 



Pdlceontological Notes. — Mr. Baily gives a list of localities from which 

 fossils have been collected, another of the various fossils so obtained, 

 and details as to the most important species. Among the Silurian 

 (Upper Llandovery) fossils he notices a specimen which, although ill- 

 preserved, appears to be identical with OhoJus Davidsoni, a species new 

 to Ireland. Several bivalve shells new to Ireland were also collected 

 in the Llandovery Beds. 



Microscoj)ical Notes. — Prof. Hull describes some of the igneous rocks 

 from Ugool. Felstone Porphyry, from near Tawnyinah woods, has a 

 mottled felsitic base, with crystals of orthoclase, also chlorite (mostly 

 structureless), grains of magnetite, with hornblende and silica, which 

 shows apparent gas-cavities, and supposed mica. Diabase, from 

 Tawnyinah woods and S. of Bellahy : the association of minerals is 

 unusual, orthoclase with augite; the rock is therefore not properly 

 either diabase or melaphyre, but appears to come under the head of 

 pyroxenite of Dana. The specimens show a mottled felsitic base, with 

 imbedded crystals of orthoclase, augite (or pyroxene), magnetite, and 

 grains of silica. Chlorite also occurs. Felstone Porphyry (Tawnyinah 

 Tower) in a felsitic base has orthoclase, a triclinic felspar, grains of 

 silica, and magnetite. Basalt (three miles S. of Bellahy), a compact 

 rock ; it consists of small prisms of felspar in an augitic base, with 

 magnetite or probably titano-ferrite, and pseudomorphs after olivine. 

 The unusual occurrence of several crystals of orthoclase is noticed, also 

 peculiar nail-like bodies apparently of the same composition as the 

 ferrite grains. 



The remainder of the memoir is occupied with the " Detailed De- 

 scriptions" of each Sheet. Red-deer horns were found in marl at 

 Lough Gal. E. T. H. 



WiLLETT, H. Report on the Sub-Wealden Exploration. Brit Assoc. 

 Rep. for 1873, pp. 490, 491. 



Gives details of the boring from the surface to 300 feet. This part of 

 the boring is 9 inches in diameter. A Geological Report, by W. Topley, 

 is appended (see p. 37). 



