Vol. 67.] Till: CARBON IFIKOTJS 8UCCE3SI0N 1-N GOWER. 515 



Limestones of a type. — Limestones of a type, named after 

 the well-known Bryozoa Bed, Horizon a, 1 of the Avon, consist largely 

 of crinoid-ossicles and dendroid bryozoa, broken and rounded by 

 rolling and reddened with haematite. Haematite may appear, also, 

 in the matrix and in the coatings of ooliths, but the characteristic 

 feature of the rocks is the relation of the iron-oxide to the 

 fossils; it infills organisms, such as bryozoa, that 

 possessed open cavities, and picks out the trabecular 

 structure of the crinoid-ossicles. 2 



The significance of limestones of a type lies in this — that their 

 distribution and lithological characters, details of which are reserved 

 for another communication, all point to the conclusions that they 

 have received their iron-content (apart from secondary enrichment) 

 at the time of their deposition, and that the iron-compound has 

 been, from the first', in the condition of oxide and probably, as now, 

 in that of haematite. These views, though differing directly from 

 those of Prof. Cayeux as to the origin of a Devonian (Eifelian) 

 haematite from the Ardennes which has the characteristic structure 

 of limestones of a type, 3 are in full agreement with the conclusions 

 of C. H. Smyth, Junr., 4 and others 3 regarding the period of intro- 

 duction and primary nature of the haematite in the well-known 

 Clinton (Silurian) iron-ore of the United States, an ore that also 

 contains a quantity of rolled bryozoan and crinoidal debris, infilled 

 and picked out with haematite. 



The contemporaneous deposition, in a sedimentary formation 

 unassociated with igneous rocks, of a considerable quantity of an 

 iron compound, — and especially of haematite, if that were the 

 original condition of the iron-ore, — appears to be incompatible 

 with open-sea conditions and to demand comparative isolation and 

 proximity to land. Limestones of a type are, therefore, directly 

 suggestive of deposition in isolated, coastal areas 6 ; but, at the 

 same time, tbeir fauna, which consists largely of standard forms, 

 and the broken and rolled state of their constituents, indicate 

 conditions which, though shallow, depart from those — of abnormal 

 fauna and gentle sedimentation, as we shall see — most charac- 

 teristic of lagoon-phases. 



Purple and green shales. — Purple and green shales that owe 

 their colour to conditions of deposition are of much more restricted 



1 Described by A. Yaughan, Proc. Bristol Nat. Soc. ser. 4. vol. i (1906) p. 99. 

 - 8ee a microphotograph of a limestone of a type in 'The Country around 

 Newport ' Mem. Geol. Surv. 2nd eel. (1909) pi. fig. 1. 



3 ' Les Minerals de Fer Oolithique de France' fasc. i (1909) pp. 225-26: 

 the structure in quesion is beautifully figured in this volume. I am greatly 

 indebted to the courtesy of Prof. Cayeux for the use of some of his material. 



4 Zeitschr. f. prakt. Geol. vol. ii (1894) pp. 304-13. 



5 For instance, E. F. Euro-hard & E. C. Eckel in ; Iron Ores, &c. of the 

 Birmingham District, Alabama ' Bull. U.S. Geol. Surv. 400 (1910) pp. 28 & 40. 



6 This view, also, agrees with that of the American observers mentioned 

 above, who regard the Clinton ore as deposited in shallow bays and lagoona 

 (see, for example, op. supra cit. p. 40). 



