256 



PROCEEDINGS OF THE GEOLOGICAL SOCIETT. 



rocks may be distinguished in Wales, most of them having exerted 

 great influence over ancient life. Thus, there are five varieties of 

 micaceous sandstone or grit, whilst there appears to be only one in 

 'New York : twelve forms of schist or shale are met with ; but lime- 

 stone undergoes only the mutations common in New York. 



In constructing the Table of sedimentary positions of the Silurian 

 fauna and flora of Wales (Table YI.)? I have not arranged them 

 under these thirty heads. By limiting them to eleven, it is hoped that 

 every practical advantage is secured, and some disadvantages avoided. 

 These Welsh strata differ from those of New York in the prevalence 

 of micaceo-siliceous sandstone, in the absence of rich beds of iron- 

 ore, and in the presence of the green colouring-matter of the mud- 

 stones. The sedimentary strata of the Welsh area have been analysed 

 in a few instances only, by Messrs. Trenham Eeeks, David Forbes, and 

 Prof. Forchhammer. These analyses are annexed, one being added 

 from Scotland and another from Norway. Those of New York have 

 not been examined ; but the late Dr. Troost, Professor of Geology at 

 Nashville College, Tennessee, kindly submitted to chemical analysis, 

 at my request, some Canadian rocks, a continuation of the basin of 

 New York. 



It is seen from Table lY., imperfect as it is, that the consti- 

 tuents of the Canadian strata are few and simple, and that they are 

 in different proportions in the same stratum at different places. In 

 the Table of Welsh Rocks (No. III.) much less silex is noticed in the 

 Lower Silurian, while in the Wenlock limestone of Dudley there is 

 often little alumina, so plentiful in the Niagara Limestone of central 

 New York. It is possible that phosphoric acid, detected in the 

 Welsh rocks by David Forbes, has not been met with in the earlier 

 palaeozoic strata of this State because it has not been looked for — 

 and perhaps for want of the detective reagent we have of late years 

 become possessed of. The phosphates are common among American 

 coal-measures, and in extraordinary abundance in the older meta- 

 morphic rocks. The want of a greater number of exact analyses 

 becomes of less importance on account of the frequent changes in 

 mineral proportions observed in almost all North American sedi- 

 mentary rocks. The Hudson-Eiver and Clinton sections vary 

 through almost all the possible mutations of sand, clay, Hme, and 

 iron. The Niagara Limestone at the west end of Lake Ontario is 

 pure, siliceous, or argillaceous ; while it is invariably magnesian 

 about the west end of Lake Huron, 500 miles W.N.W., probably 

 on account of the proximity of hypogene rocks. Calciferous sand- 

 stone, reported by Emmons to be free from magnesia in Jefferson 

 County, New York, is, according to Foster, rich in that mineral on 

 the south shore of Lake Superior. 



As regards the Devonian system of New York, the changes in the 

 calcareous rocks of its lower stage and in the shales of the middle 

 stage are commonly far too numerous and too gradual to admit of 

 useful quantitative analysis. These observations apply also to 

 Wales*. 

 * The subject of mineral deposition is most ably treated in the writings of 



