DAVIDSON — SCOTTISH CARBOXIFEROUS BRACHIOPODA. 



463 



general reader may better understand the position in the series occu- 

 pied by the species to be hereafter described. 



It is well known to every one possessing a knowledge of the first 

 rudiments of geology, that the Carboniferous system lies between 

 the Devonian and Permian formations ; but we cannot expect always 

 and everywhere to find the sequence complete. Instances are not 

 wanting wherein Carboniferous strata repose directly upon Silurian 

 or older rocks, and are overlaid by Jui'assic or still younger deposits ; 

 but in such cases, which are likewise common to formations of all 

 ages or periods, the natural order of succession does not exist, for the 

 strata which should underlie or overlie in natural order are wanting 

 from some cause or another. 



The rocks which compose the Carboniferous series are not every- 

 where exactly similar ; for in some districts a certain bed, or series 

 of beds, may be largely developed, while they may be attenuated, or 

 entirely absent, in another. The Carboniferous system is made up 

 of a vast accumulation of conglomerates, sandstones, shales, iron- 

 stones, limestones, and coal-seams, and certain portions of the system 

 present a marine, others an estuary character, while a third is entirely 

 composed of terrestrial vegetation ; and, as stated by Mr. Page, in 

 his excellent text-book, " the frequent alternations of strata, and the 

 great extent of our coal-fields, indicate the existence of vast estuaries 

 and inland seas, of gigantic rivers and periodical inundations, while 

 the mountain-limestone, with its marine remains, reminds us of low 

 tropical islands fringed with coral-reefs, and lagoons thronged with 

 shell-fish and fishes." 



Before alluding to the divisions that have been proposed for the 

 Carboniferous system, we must briefly notice that, although Sir R. 

 Murchison and the generality of geologists have pronounced the Old 

 Red sandstone of Scotland* to be the full equivalent in time of the 

 Devonian rocks of other countries, some geologists, such as Professor 

 de Koninck and Mr. Kelly, have suggested that these red sandstones, 

 especially in the southern portion of Scotland, and in Ireland, should 

 be considered as forming part of the Carboniferous series. Be this 

 as it may, the celebrated author of the " Silurian System " has him- 

 self admitted that, " as we approach the summit or higher beds of 

 the Devonian or Old Red sandstone, we are gradually introduced to 

 the fauna of the Carboniferous era." Therefore the passage between 

 the upper beds of the Old Red sandstone and the lower one of the 

 Carboniferous system has been gradual ; while the same may be said 

 relative to that w^hich connects the upper beds of the Carboniferous 

 with the lower ones of the Permian formations. Some geologists 

 would, therefore, place the basement-line of the Carboniferous system 

 lower down than others appear disposed to admit ; and hence the 

 difference in opinion that may be traced in the various sections or 



* The rock is not everywhere of a red colour, there being also enormous beds 

 of yellow, whitish, pnrj^lish, and rusty-coloured sandstone, with coarse con- 

 glomerates, and dark-grey micaceous flagstones. 



