ORIGIN OF THE UPPER COAL MEASURES. 



97 



the same microscopic shell, Microconchus carhonarius. The subsequent discovery by 

 which the limestone of Ardwick, near Manchester, was identified with it, has given to 

 this stratum a considerable additional importance, in carrying out over so wide an area 

 the evidences adduced in this volume of the passage of the coal measures beneath the 

 New Red Sandstone of the central counties 1 . Besides the zoological proofs of this lime- 

 stone having been formed in fresh water, I have already stated, that in mineral characters 

 it strongly resembles the lacustrine limestone of central France, and I may now add 

 that the origin of the rocks in the two countries is probably connected with similar 

 causes. For as Auvergne is a region which has been eminently subjected to volcanic 

 action during past ages, so its extensive formations of finely levigated limestone are sup- 

 posed to have been the produce of hot springs (the usual attendants on volcanos), holding 

 calcareous matter in solution, and depositing it amid the fine silt of ancient lakes. In 

 like manner the whole of the surrounding region of Shropshire, in which this limestone 

 occurs, is absolutely perforated by intrusive rocks of igneous origin, (see Map) ; and 

 hence it is a fair deduction, that the peculiar limestone of this tract may likewise have 

 been the result of volcanic hot springs. Other analogies will strike those to whom the 

 phenomena in central France are familiar, such as bituminous exudations and sources 

 of mineral pitch which issue from the surface at those points where eruptive rocks pro- 

 trude ; but these comparisons belong more properly to subsequent chapters. Difficult as it 

 may be to reconstruct in imagination the condition of the surface of this part of our 

 island during the period of the coal formations, the limestone and associated beds 

 (whether formed exclusively in pure fresh water or in bays in which fresh predominated 

 over salt water) afford convincing proof of the existence of neighbouring dry land, from 

 which rivers flowed, transporting terrestrial vegetable remains, and entombing them with 

 shells, the greater part of which must, unquestionably, have lived in fresh water. That 

 such streams, however, were near the sea, and that in fact they soon passed into estua- 

 ries, will be presently rendered evident by details of the undeniable alternation and in- 

 termixture of freshwater, terrestrial, and marine remains in Coal Brook Dale, which 

 tract, though only distant a few miles from that under consideration, exhibits a vast 

 expansion of the carboniferous strata; thus leading us to suppose, that whilst the Shrews- 

 bury deposit has been simply formed by streams issuing from the Cambrian and Silurian 

 region, and giving rise to lakes to which the sea had little or no access, the greater car- 

 bonaceous masses of Coal Brook Dale have been accumulated by the same waters where 

 they united to empty themselves into an estuary 2 . The north-eastern edges of this 



1 Mr. Greenough informs me that he has observed a bed of limestone interstratified with coal in Warwick- 

 shire which he considers to be of the same age, but he has not observed any organic remains in it. 



2 A point of high comparative interest attached to these coal-fields of the central counties, is that they 

 contain fossil fishes, mollusks and entomostraca, identical with or closely allied to species abounding in the 

 rich fossil accumulation at Burdie House near Edinburgh, the description of which by Dr. Hibbert justly 

 excited so much attention. Though not, perhaps, quite so copiously charged with] organic remains as 



