236 Geological Society. 



(4)?Bala Beds. — Coarse conglomerate and Bandstone containin 

 pebbles, mainly of granite and felsite. 



(3) Llandeilo Beds. 



(c) Shangort Beds. — Grits and tuSs, coarse and fine, the prevalent 

 type being a calcareous gritty tuff, in wiiicli is a series of 

 limestone-breccias, having a inaxiniuui thickness of about 

 40 feet and largely formed of disrupted fragments of the 

 underlying limestone. 



(5) Tourmakeady Beds. — Compact pink, grey, or white limestones, 



sometimes in beds with a maximum thickness of about 30 feet, 

 but usually represented by blocks in the Shangort Beds. 

 {a) Red felsite or rhyolite. — A series of flows varying much in 

 thickness. 



(2) Are nig Beds — Mount-Partry Beds. 



{d) Variable tuffs, grits, and cherts, the tuffs being seen only in tha 



southern half of the area, 

 (c) Coarse quartzose and felspathic grits. 



(6) Grits, graptolitic black slates, and radiolarian cherts. 



(a) Coarse conglomerates, the pebbles of which consist almost 

 entirely of grit. 



A considerable series of graptolites, collected from the Mount- 

 Partrj' Beds, has been examined by Miss G. L. Elles, D.Sc, and 

 they prove to be of Upper Arenig age — about the zone of 

 Didytnograptus hirundo. The radiolaria from the same series 

 of rocks have been studied by Dr. G. J. Hinde, F.E.S. 



The most interesting and puzzling beds of the district are those 

 of Llandeilo age. Although the limestones (Tourmakeady Beds) 

 occur in the main as disrupted blocks in the gritty tuffs (Shangort 

 Beds), the fossils indicate that there is no material difference in the 

 age of these two deposits ; and the Authors believe that, after the 

 deposition and consolidation of the limestone, but during the pre- 

 valence of the same faunal types as those which characterize that 

 deposit, the limestone was broken up by volcanic explosions, and its 

 fragments, mingled with bits of felsite and other material, were 

 deposited as the peculiar limestone-breccias. This view regarding 

 their formation is held to afford an adequate explanation of the 

 patchy development of these rocks. 



The intrusive rocks are of considerable interest. They are, in the 

 main, felsites with large quartz-crystals, and not infrequently 

 contain augite. Some of them are certainlj- intrusive in the coarse 

 Bala (?) conglomerate. A number of small but interesting intrusions 

 of olivine-dolerite, hornblende-lamprophyre and line-grained oligo- 

 clase-bearing rocks are scattered throughout the district. 



The appendix embodies a critical review of the fauna of the Llan- 

 deilo Beds of the district, and a description of several new species of 

 brachiopods and trilobites. 



