128 MR. F. DIXET AXD PEOF. T. F. SIBLY OX THE [vol. IxXlii, 



The hillside excavation (Grlam. 37 SW X3), in the southern 

 limb o£ the Tons^wynlais syncline, yields the foUowino; section : — 



2. Green-grey shales : upper part ^\T.th many thin layers of fine-grained, 

 highly gritty limestone yielding ostracods and brachiopods. The limestone- 

 layers extensively decalcified to a fine, ferruginous sand 



to top of section 6 feet. 



1. Crinoidal limestones : grey limestones ; but at the top, a band of 

 li^ematitic limestone of a-tj-pe, up to 10 inches in thickness, and at the base, 

 limestone of the same t^^pe, with partings of green shale, seen for 2 feet. 

 Abundant brachiopods (Prochictus hassus, Chonetes cf. liardrensis, Leptx)ia 

 analoga, Camarotoechia mitcheldeanensis, etc.) ... to base of section 12 feet. 



The railway-cutting (Glam. 37 SW A.4) lies in the northern 

 limb of the syncline. It extends southwards from Castell-Coch 

 Tunnel, and traverses a considerable portion of the Lower Lime- 

 stone Shales. For the most part it lies in the middle group of 

 crinoidal limestone and oolite, but at the northern end it exposes 

 the underlying limestones and shales to a thickness of about 

 20 feet. These latter beds are highly fossiliferous, _yielding all 

 the types of our faunal list, above. Chonetes cf. liardrensis teems 

 in many bands of limestone. Froductiis hassus is abundant, 

 Chonetes stoddarti rare. Cleistopora cf. geometrica occurs in 

 the shah" cappings of some limestone-bands. The strata comprise 

 limestones and shales interbanded in lenticular fashion, and a 

 considerable development of argillaceous limestone. The shales, 

 grey to black, calcareous and micaceous, occasionally contain 

 numerous ostracods or Fenestella. with a few lamellibranchs or 

 small Chonetes. The limestones are predominantly dark-grev 

 shelly rocks, with an abundance of brachiopods and ostracods, 

 though seldom conspicuously crinoidal. But this shellj type of 

 limestone is intimately interbanded with very fine-grained, poorly 

 fossiliferous limestone, sometimes smooth and splintery, sometimes 

 laminated and gritty. 



These sections in the Taff valley exhibit a development of shale 

 equal or nearly equal in amount to limestone, a considerable 

 change from the character of the group in the Ebbw valley. 

 Limestone of a-type is also developed in larger amount, but 

 evidently at the same horizon. In the neighbourhood of Ton- 

 gwynlais, moreover, the limestones of a-type have undergone some 

 secondary enrichment in hiematite, and this has led to more than 

 one trial of the beds for iron-ore. Excellent specimens for the 

 study of these rocks may be obtained, either from the excavation 

 at Tongwynlais (Grlam. 37SWX3) described above, or from the 

 spoil-heap of an old level north of Khubina (Grlam. 37SWX11), 

 mentioned in the Gleological Surve}" memoir.^ Specimens E 246-> 

 & E 2166 of the Geological Survey collection were obtained from 

 the latter locality. E 2465, illustrated by a microphotograph 

 in Plate, hg. 1, of the memoir, possesses a matrix of clear, 

 gi-anular calcite. comparatively free from hsematite : in E 2166, on 

 the other hand, the matrix has suffered much ferritieation. In 



^ Newport Memoir, 2nd ed. p. 24. 



