442 



SPECIAL GEOLOGY. 



mites, with here and there a larger tree. The spoils of these 

 plants may have furnished materials for beds of peat and 

 coal, resting on a base composed of the remains of Stigmaria. 

 The lagoon or the morass, by repeated subsidences, may 

 have sunk beneath the level of the sea, and have rendered 

 the basin the receptacle of alternating deposits of sand and 

 clay, and may thus have produced the strata of sandstone 

 and limestone which occur between seams of coal. As each 

 deposit was formed, it may have been covered either wholly 

 or in part by a lagoon, when the same succession of vegetable 

 growth and deposit may have ensued. 



The alternation of beds of coal with marine deposits is 

 explained by the supposition that an extensive subsidence of 

 the estuaries, which were the site of the lacustrine and 





FIG. 296. Section showing the erect position of fossil trees in coal sandstone 

 at St. Etienne. (A. Brongniart.) 



terrestrial vegetation, may have reduced them beneath the 

 level of the sea, where the submerged soil with its vegetation 

 was covered with marine sediments ; in course of time, either 

 by drifts of sand or clay from the land, or by the elevation 

 of the bed of the sea, the estuaries were again filled, and 

 became the area of the vegetable growth above named, while 

 the repetition of such changes would account for the alter- 

 nation of marine and lacustrine strata which occur in beds of 



