312 Mr Horner ow the occurrence of' the Megalichthys 



must have constituted the substance of pre-existing rocks, which 

 were abraded by atmospheric agencies and running water, the 

 detritus being afterwards transported by rivers to the sea ; and 

 in some of the beds thus formed, such as the coal-measures, the 

 products of fresh-water are in great abundance. But this is not 

 the sense in which Dr Hibbert employs the term : he considers 

 the bed of limestone in question to have peculiar distinctive cha- 

 racters ; that he has made a discovery of a new feature in our 

 coal-fields, and one, moreover, which he had been long expect- 

 ing to find. " 1 had long," he says, " been prepared to expect 

 that a limestone of a fluviatile or a fresh-water origin would, 

 some time or other, be proved to exist.'' — P. 169- He states 

 (p. 267) " that it must have been the result of a deposit in fresh- 

 water, hostile to the growth and increase of marine shells and 

 corallines ;" that this limestone bed " indicates some fresh-water 

 river or lake, within which calcareous matter was elaborated. — 

 P. 253, Farther, that " the beds of argillaceous shale, both above 

 and below, enclose the same organic remains as are found in 

 the limestone, along with coprolites, shewing that they are them- 

 selves a portion of the lacustriiie deposit of this locality." — 

 P, 244. And, at p. 272, he says, " Hitherto, however, I have 

 not found the slighest traces of marine mollusca or corallines in 

 the limestone of Burdiehouse ; and hence, I am not induced 

 to consider it as any thing but a pure lacustrine Jbrmationy 



It is now generally admitted, as the most probable theory of 

 the formation of coal-deposits, where there are interstratified 

 marine beds, that they have taken place in estuaries, in those 

 deep indentations of the land which often occur at the mouths 

 of great rivers ; and where the beds that are gradually formed, 

 by the subsidence of the solid materials brought into it by the 

 waters, must contain the productions both of the sea and land ; 

 those of the land, however, naturally predominating. The beds 

 of coal are usually considered to have been formed by the accu- 

 mulation of large quantities of vegetable matter, drifted into the 

 estuary from the land, and deposited upon a previously formed 

 surface of sand, clay, and mud, indurated afterwards into stone 

 by pressure, and by a chemical action among the particles, in- 

 duced by that enormous pressure ; the vegetable matter being 

 converted into coal by the combined chemical action of water 



