Vol. 65.] ROCKS OF THE TOURMAKEADT DISTRICT. 139 



the extreme northern part of the area and in the Mount Partry 

 and St. Mary's Monastery district. They consist of closely packed 

 lapilli of felsitic, and occasionally of andesitic rocks, with little 

 or no gritty material. Chlorite tends to be plentiful, sometimes 

 forming irregular patches between the lapilli, sometimes probably 

 representing altered pyroxenes. Unaltered pyroxenes are also 

 present among the constituents of the matrix. 



Only rarely, as, for example, at (306) south-east of the brown 

 felsitic intrusion of Shangort, were tuffs free from admixture with 

 gritty material found in other parts of the area. 



(3) The limestone- breccias. — -Microscopical sections of 

 these rocks show little variation at the various exposures, and 

 consist of a calcareous matrix through which are scattered quartz- 

 grains and ashy particles in varying proportions. The matrix 

 consists principally of limestone, of a horny or not well-cleaved type ; 

 but many irregular patches of well-cleaved calcite are also invariably 

 present. The lapilli, while predominantly of quartz-felsite, are 

 sometimes of andesitic rocks (45). 



(4) The coarse felsite-breccias or agglomerates. — The 

 matrix of these rocks is the same as in the finer non-calcareous 

 types of tuff. 



V. The Volcanic Tent or Neck. 



At a short distance south-west of Shangort occurs an oval mass 

 of coarse breccia having a length of about 100 yards, which Sir 

 Archibald Geikie (' Anc. Vole. Gr. Brit. vol. i, 1897, p. 253) regards 

 as marking a vent, a view with which our own observations are 

 in agreement. This is the only mass which we can claim with 

 confidence as marking the position of a vent or neck. 



VI. Summary and Conclusions. 



"We believe the general succession of the Ordovician rocks of the 

 district to be as follows : — 



3. (?)Bala Beds. 



Coarse conglomerate and sandstone, containing pebbles mainly of granite 

 and felsite. 



2. Llandeilo Beds. 



(b) Shangort Beds.— Grits and tuffs, coarse and fine — the prevalent 

 type being a calcareous gritty tuff. In this tuff occur bedded lime- 

 stones and limestone-breccias, the Tourmakeady Beds. The 

 bedded limestones have a maximum thickness of about 30 feet, and 

 the limestone-breccias of about 40 feet. The latter rocks are largely 

 formed of disrupted fragments of limestone corresponding in litho- 

 logical character to the bedded limestone. 



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

 thickness. 



