Vol. 65.] BOCKS OF THE TOTJKMAKEADY DISTRICT. 123 



South-west of Shangort is a mass of compact limestone, having a 

 probable thickness of not less than 40 feet and a length of about 100 

 yards. This mass (88), which is exposed in a rather large quarry, 

 is faulted against red felsite on the east. Very small patches 

 of Carboniferous conglomerate are seen in the quarry resting upon 

 both rocks. Lithologically the limestone is hard, compact, and 

 horny, usually grey but sometimes pink. It frequently has small 

 quartz-grains thickly scattered through it, and may be said to pass 

 in places into a calcareous sandstone. Here and there patches of 

 red chert occur in the limestone. Fossils are very scanty ; several 

 hours' search yielded only one Ortliis simplex, M'Coy, a fragment of 

 Harpes, and a crinoid-joint. 



North-west of Shangort is a strip of limestone-breccia having a 

 length of about 150 j^ards, and trending in a north-easterly direction 

 along the course of a small tributary of Stream H. The matrix of 

 the breccia is the usual type of gritty ash, and the fragments seem 

 to be exclusively of limestone, no felsite having been found. 



As the fossiliferous limestone generally occurs in the form of 

 dislocated blocks, the deposit as a whole was originally regarded 

 by the officers of the Geological Survey as of ' Upper Silurian ' 

 age, the blocks being considered derivative : but Sir Archibald 

 Geikie & Mr. Kilroe (Ann. Eep. Geol. Surv. for 1896, p. 49) 



*■ obtained clear evidence that, the limestone is truly interstratifiecl in the vol- 

 canic series ; that the fossils it contains are not derivative, but belong to the 

 time of deposition of the limestone, and tbat the same organisms occur in the 

 calcareous tuffs associated with the limestone.' 



Sir Archibald Geikie writes as follows with regard to the calca- 

 reous beds found in the Tourmakeady, Glensaul, and Lough Xafooey 

 districts (' Anc. Vole. Gr. Brit.' vol. i, 1897, p. 252) :— 



' The rocks in each of these three areas are similar. One of their distin- 

 guishing features is the intercalation among them of a fossiliferous limestone 

 and calcareous fossiliferous tuffs, which contain well-preserved species of 

 organisms characteristic of the Bala division of the Lower Silurian rocks. 

 There cannot be any question that these organisms were living at the time the 

 strata in which their remains occur are found [sic].* The most delicate parts 

 of the sculpture on Elwnus Boivmanni and Orthis elvijantula are well preserved. 

 Nor have the limestones been pushed into their present places by volcanic agency, 

 or by faults in the terrestrial crust. They are not only regularly intercalated 

 among the volcanic rocks, but the limestone in some places abounds in volcanic 

 dust, while above it come calcareous tuffs, also containing the same fossils. 



It is thus clearly established that the volcanic series has its geological 



age definitely fixed as that of the Bala period.' 



[* (?) Misprint for ' were formed.'] 



"With regard to the age of the limestone, Mr. Reed is convinced 

 by the fossil evidence that it is not of Bala, but of Llandeilo age. 

 The reasons for this view are set forth in the appendix to this paper. 

 Further, in the passage quoted above, Sir Archibald Geikie appears 

 to imply that all the limestones lie associated with the tuffs approxi- 

 mately as they were originally deposited. This is no doubt the 

 case with regard to the compact bedded limestones, but it appears 

 to us impossible to maintain this view as regards the rocks which 



