Vol. 62.] CAEBONIFEKOUS HOCKS AT RUSH. 285 



The most conspicuous of the conglomeratic bands usually vary from 

 2 to 4 feet in width, while one [R 10 K] is nearly 12 feet thick. 

 They dip steadily northward in conformity with the underlying 

 beds at angles of 50° to 65°, and they are broken by numerous small 

 faults, which are well displayed on the shore close to Rush Harbour 

 (see fig. 6, p. 287). The included pebbles are, as a rule, fairly well 

 water-worn, their angles and frequently their whole surface being 

 rounded. In some bands the inclusions are small, not exceeding the 

 size of a hazel-nut ; in others they are considerably larger ; while in 

 some beds very large pebbles abound, frequently exceeding 1 foot in 

 length. One pebble measured 26 X 22 inches, another 32 x 14 x 10 

 inches, and even larger ones could probably be found by more 

 exhaustive search. They consist for the most part of white quartz, 

 grits and slates of various colours, andesitic rocks, and limestone, 

 and are usually embedded in a limestone-matrix in which corals, 

 brachiopods, crinoid-fragments, and other fossils are not infrequent. 



With the exception of the limestone-inclusions, the origin of the 

 pebbles is easily referred to the Ordovician and Silurian rocks of 

 the neighbourhood. Such rocks are now found in situ at Portraine 

 on the south, and near Skerries on the north. The limestone-pebbles, 

 which predominate largely in some of the beds, present more 

 difficulty. Although it is possible that a few of them m ay be derived 

 from the Ordovician limestone of Portraine, the very great majority 

 are undoubtedly of Carboniferous-Limestone age. Now, the Rush 

 Conglomerates and the Pendine Conglomerate of South Wales are 

 shown, by the zonal work of Dr. Yaughan, to be on the same 

 horizon (Syringothyris-Zone). The latter conglomerate also contains 

 Carboniferous-Limestone pebbles, which are thought, from fossils 

 found in them, to have * been derived from a horizon probably not 

 higher than a low part of the Zaphrentis-TAOne? l To regard the 

 Rush Conglomerate-Group, like the Pendine Conglomerate, as an 

 ' intra-formational conglomerate ' 2 in respect to its limestone- 

 pebbles, seems to be on the whole the most satisfactory explanation 

 of the presence of these inclusions. The few fossils found in 

 them have, as yet, yielded no clear evidence as to the horizon from 

 which they have been derived ; but, so far as it goes, it implies very 

 little or no difference of age. The occasional presence of small 

 pebbles in these inclusions suggests, however, that they may be of 

 contemporaneous or almost contemporaneous horizon, or may in 

 some cases have been formed in situ. One limestone-inclusion, 

 occurring as a lenticle 7 feet long by 4^ feet wide, in a bed of very 

 pebbly limestone [R 11/], must certainly have been formed in place. 



The conglomerates have greatly interfered with the action of 

 the cleavage that has had so marked an effect on the Rush Slates. 

 Not only are they quite unaffected by it themselves, but they have 

 almost completely protected the beds intercalated in them, even the 



1 ' Summary of Progress of the Geological Survey for 1904 ' Mem. Geol. 

 Surv. 1905, p. 44. 



2 Cf. O. D. Walcott, ' Palaeozoic Intra-Formational Conglomerates ' Bull. 

 Geol.Soc. Amer. vol. v (1894) pp. 191-98. 



Q. J. G. S. No. 246. 



