Vol. 65.] ROCKS OF THE TOURMA.KEADY DISTRICT. 105 



Lough Mask. A number of streams traverse the district from west 

 to east, and afford an excellent series of sections ; but, apart from 

 these, exposures are almost everywhere numerous, this being partly 

 due to the fact that the district is free from drift. 



Considering the interesting and varied character of its geology, 

 it is remarkable that comparatively little has hitherto been written 

 about this area. In 1871, Gr. H. Kinahan, 1 in a paper on the ' Geology 

 of West Galway & South- West Mayo,' while in the main dealing 

 with districts farther south, incidentally refers to the Tourmakeady 

 district. 



By far the fullest account of the district is that by G. H. 

 Kinahan & II. G. Symes, contained in the Explanation of Sheets 73, 

 71 (in part), 83, & 81 of the Geological Survey map of Ireland, 

 published in 1876. We shall frequently have occasion to refer to 

 this, but will here merely mention that the igneous series, as a 

 whole, is considered to be of Silurian (Upper Silurian) age, the 

 remarkable limestone- masses with Ordovician fossils, to be described 

 later, being regarded as dislocated blocks which had been carried up 

 by later volcanic eruptions. 



Nothing appears to have been written about the district for the 

 next 20 years, but the Annual Report of the Geological Survey for 

 1896 2 contains an account by Sir Archibald Geikie of Mr. J. R. 

 Kilroe's work, in which the occurrence is mentioned of pre-Bala 

 fossiliferous rocks, and the whole igneous series is considered to be 

 not of Silurian (Upper Silurian) but of Ordovician (Lower Silurian) 

 age. This conclusion is farther elaborated in Sir Archibald Geikie's 

 * Ancient Volcanoes of Great Britain,' 3 where a generalized section 

 across the area is given. 



Finally, the paper by Mr. Kilroe, ' On the Silurian & Meta- 

 morphic Rocks of Mayo & North Galway,' recently ^published, 4 

 contains a number of references to the district with which we are 

 dealing. The most important point in Mr. Kilroe's paper, however, 

 is the conclusion that the coarse grits and conglomerates, which 

 all previous observers had regarded as forming a single group, 

 really include two groups, one of Llandeilo or Arenig, the other of 

 Bala age. 



One of the disadvantages, with which a geologist engaged in 

 mapping this district has to contend, is the difficulty of clearly and 

 briefly describing the position of any spot, owing to the small 

 number of names inserted in the 6-inch Ordnance maps, the names 

 of hamlets given being actually fewer than in the 1-inch Geological 

 Survey map. We have, therefore, found it convenient in this 

 paper to distinguish the various streams by the letters B, C, etc. 



1 Geol. Mag. n. s. dec. ii, vol. i (1874) p. 453. A paper on ' Geological Maps 

 & Sections of West Galway & South- West Mayo ' by the same author was 

 read at the British Association Meeting at Belfast (1874), but only the title 

 appears in the Eeport (Trans. Sect. p. 88). 



2 Ann. Eep. Geol. Surv. U. K. for 1896 (1897) pp. 49-50. 



3 Vol. i (1897) pp. 251 etseqq. 



4 Proc. Eoyal Irish Acad. vol. xxvi, sect. B, no. 10 (1907) pp. 129-60. 



