20 DB. STAMP AND ME. WOOLDEIDGE ON THE [vol. lxxix, 



only a few large vesicles. The underlying beds are not exposed, and, 

 owing to the strong southward pitch of the anticline, it is difficult 

 to estimate the thickness of the spilites. They appear, however, 

 to be certainly not less than 40 or 50 feet thick, and may be much 

 more. Beautifully fresh examples of the rock — here of a pale 

 greenish colour with inconspicuous vesicles — may be collected from 

 the bed of the River Irfon. The spilite forms reefs crossing the 

 river obliquely, and, when the water is low, the little bands of 

 chert separating the 'pillows' are well seen. 



As we pass to the transverse valle} r of the Nant Cerdin, the 

 spilites may be traced almost continuously on both sides of the 

 valley, and show at once the anticlinal structure of the mass. In 

 the upper part of the band marked as spilite on the map (PI. II), 

 two flows, each a few feet thick, may sometimes be distinguished. 

 Both consist, for the greater part, of typical pillow-lava ; and the 

 two are separated by a few feet of ashy sediments. The interest of 

 this section, however, lies in the development below the spilite-flows 

 of a curious rock which may be described as a ' spilite-breccia.' 

 The term is not a new one, having been used before by Mr. C. I. 

 Gardiner & Prof. S. H. Reynolds. 1 The rock consists of a typical 

 breccia or coarse ash, in which are embedded innumerable ' bombs * 

 of spilite. These ' bombs ' resemble miniature pillows ; they are 

 vesicular within, and possess a thin glassy crust. They vary in size 

 from about 1 to more than 12 inches in diameter. As the spilite- 

 ' bombs ' increase in size and number, the rock becomes indistin- 

 guishable from an ordinary spilite-flow. When weathered, the 

 rounded outlines of the spilite-' bombs ' contrast strongly with the 

 roughened angular appearance of the ashy matrix. This rock is 

 not easily found in situ ; but numerous blocks are scattered on the 

 southern slopes of the Nant- Cerdin Valley, and have been built 

 into the wall which crosses that valley from north to south (see- 

 map, PI. II). Farther north, the crags marked D on the map 

 exhibit other examples of spilite-breccia. It is interesting to note 

 that a very similar rock has been described from Jersey : 2 there 

 again the breccia passes gradually upwards into true lava-flows. 



Ecpually curious is the rock which constitutes the core of the 

 anticline on the north, forming the central ridge of Car Cwm. 

 The main mass of the rock presents a distinctive appearance ; it is 

 dark greenish-grey, with numerous black specks about 1/8 inch 

 in diameter. These specks represent vesicles infilled with a dark 

 chloritic material. The base of this rock is not seen, and its 

 relations with the overlying beds are not easily determined. It 

 seems, however, to become more vesicular in the upper part, and to. 

 pass gradually upwards into typical pillow-lavas. There is thus a 

 fringe of typical pillowy spilites and interbedded cherts surround- 

 ing the central mass, which I regard as the massive lower part of 

 the spilite-flow. 



1 ' The Ordovician & Silurian Bocks of the Kilbride Peninsula (Mayo) ' 

 Q. J. G. S. vol. lxviii (1912) p. 75. 



2 T. G. Bonney & C. A. Baisin, ' On the so-called Spilites of Jersey ' GeoL 

 Mag-. 1893, p. 59. 



