478 Prof. Sedgwick on the May Hill Sandstone y 



1. Immediately on the west side of the Silurian hills is the 

 rugged trappean ridge which forms the base of the section. 

 Some parts of it are highly cellular and amygdaloidal ; and if it 

 were originally submarine, it must have been formed at least in 

 a very shallow sea. 



2. Next follows a remarkable series of beds, which towards 

 the north become greatly expanded, and which on the sectional 

 line are of considerable thickness. They are made up of a kind 

 of trappean breccia, and of a fine granular recomposed trap, in 

 some places forming a good building-stone. And these masses 

 alternate with some beds (especially in the more northern parts 

 of the range) to which the name ash might, I think, be fairly 

 given — though I greatly limit the meaning of the word ashy as 

 used in the Government Survey. In its place I formerly used the 

 term plutonic silt, and afterwards the German term schaalstein ; 

 and in the same sense I now generally use the words trap-shale ; 

 defining thereby a vast series of slaty recomposed rocks derived 

 chiefly from submarine eruptions, which therefore never existed 

 as a time ash. Alternating with such masses are also found several 

 bands of a true shale and flagstone, evidently forming a part of 

 the regular aqueous flagstone of Builth. All the above beds 

 occasionally contain fossils, among which I may mention the 

 Asaphus tyr annus, of which a noble specimen was found in one 

 of the plutonic grits. 



3. Over the above, on the sectional line, is a considerable series 

 of beds of dark shale and flagstone in which the igneous recom- 

 posed rocks almost entirely disappear. It was from this series 

 (about a quarter of a mile east of Pen Cerrig) that in 1846 I 

 collected, along with my fellow-labourer John Ruthven, a fine 

 suite of fossils, some species of which were new. They are in- 

 cluded in the list of 24 species of Builth fossils given by Pro- 

 fessor M'Coy (Camb. Palseozoic Fossils, p. 354). The locality 

 is stated to be three miles north of Builth, but the true distance 

 is not much above two miles. 



The series ends just below Pen Cerrig House, where it is over- 

 laid (as seen near a pond and small water-course) by a well- 

 defined bed of recomposed trappean rock, also containing traces 

 of fossils. 



4. Next follow calcareous and sandy beds, apparently in per- 

 fect parallelism to the former. They are not more than 8 or 10 

 feet in thickness, and the more calcareous portions are good 

 types of the Norbury limestone, and contain innumerable casts 

 of the characteristic Pentamerus. 



5. Over the preceding, and with the same strike and dip, are 

 beds of shale and earthy calcareous flagstone, with irregular len- 

 ticular concretions parallel to the bedding. These dip down the 

 water-course and form the highest beds visible in the section. 



