432 Geological Society. 



ive subsidences, have been so reduced below the level of the water, 

 as to make them the receptacles of alternating deposits of sand and 

 clay (now converted to strata of sandstone and shale) between the 

 several beds of incipient coal. During these processes, successive 

 series of lagoons may have covered large portions of each last- 

 formed drift ; and every lagoon becoming the site of a renewed 

 growth of Stigmarioe, may tims continuously have been laying the 

 foundation and nourisliing the materials of future beds of inestima- 

 bly precious fuel. 



In the case of beds of coal that alternate with marine deposits, it 

 has been suggested that extensive subsidence of tlie estuaries on 

 wliich lacustrine and terrestrial plants M'ere growing, may have re- 

 duced these estuaries below the level of the sea, where the sub- 

 merged strata of vegetable matter became covered witli beds of en- 

 crinal limestone and other marine sediments ; and that as these re- 

 ceived upon their surface furtlier sediments of sand and mud 

 drifted by land-floods into the salt-water, the estuaries were gradu- 

 ally filled up, and again converted into lagoons, upon which a re- 

 newed growth of lacustrine and land plants forthwith began to ac- 

 cumulate the materials of other beds of coal. 



Botli in the marine and the freshwater strata that alternate with 

 the coal-beds, we appeal to the tliree same intermitting and alternate 

 processes of subsidence, drift, and vegetable growth; the subsidence 

 being in the former case to a depth below the level of the sea, in 

 the latter case to a depth which left the last-formed strata in a po- 

 sition to become the site of vast swampy flats and shallo;v lagoons. 

 In both cases intermitting accunmlations of the earthy materials of 

 the strata over the subsided districts are referred to the transport of 

 sand and mud by powerful land-floods over areas wliich by subsi- 

 dence had acquired a place that made them receptacles of the de- 

 tritus of distant mountains ; as we now see vast sheets of sediment 

 transported from the Rocky Mountains and spread over the great 

 flats and vast estuaries of the Red River, the Missouri, and the 

 Missisippi. The regions on which these ancient alternations of salt- 

 water and fresh-water deposits were going on, must in the mean 

 time have presented extensive surfaces that were periodically os- 

 cillating between small distances above and below the level of the 

 Bea. 



The concentric rings of growth which may be counted in a trans- 

 verse section of the large coniferous trees whose roots are found 

 resting on the upper surface of a coal-bed, may be quoted as evi- 

 dence of the time during which it was fixed in this its place of 

 growth ; and as such trees may probably be found on the surface of 

 many successive beds in the section of a coal-field, each stage of 

 trees afibrds a chronometer by which we may calculate the number 

 of years that intervened between the growth of each bed of coal. 



In the Newcastle collieries, after the excavation of the coal, short 

 trunks of trees drop down frequently from the roof of the mine, 

 leaving vertical cavities, M'hich the miners call pot-holes; these trees 

 probably grew upon the surface of the vegetable mass by which the 



