330 OOLITE AND LIAS [On. XXI 



sea has grown shallower, or when the land, increasing in extent, whether 

 by upheaval or by sediment filling up parts of the sea, has approached 

 nearer to the spots first occupied by fine mud. 



In order to account for another great formation, like the Oxford clay, 

 again covering one of coral limestone, we must suppose a sinking down 

 like that which is now taking place in some existing regions of coral 

 between Australia and South America. The occurrence of subsidences, 

 on so vast a scale, ma}'- have caused the bed of the ocean and the adjoin- 

 ing land, throughout great parts of the European area, to assume a 

 shape favorable to the deposition of another set of clayey strata ; and 

 this change may have been succeeded by a series of events analogous to 

 that already explained, and these again by a third series in similar order. 

 Both the ascending and descending movements may have been extremely 

 slow, like those now going on in the Pacific ; and the growth of every 

 stratum of coral, a few feet of thickness, may have required centuries 

 for its completion, during which certain species of organic beings disap- 

 peared from the earth, and others were introduced in their place ; so that, 

 in each set of strata, from the Lias to the Upper Oolite, some peculiar 

 and characteristic fossils were imbedded. 



Oolite and Lias of the United States. 



There are large tracts on the globe, as in Russia and the United States, 

 where all the members of the oolitic series are unrepresented. In the 

 state of Virginia, however, at the distance of about 13 miles eastward 

 of Richmond, the capital of that state, there is a regular coal-field oc- 

 curring in a depression of the granite rocks (see section, fig. 421), which 



Fig. 421. 



Section showing the geological position of the James Eiver, or East Virginian Coal-field. 



A. Granite, sneiss, &c. B. Coal-measures. 



C. Tertiary strata. 1>. l>x\fi oi- aricient alhnmim. 



Professor W. B. Rogers first correctly referred to the age of the lower 

 part of the Jurassic group. This opinion I was enabled to confirm after 

 collecting a large number of fossil plants, fish, and shells, and examining 

 the coal-field throughout its whole area. It extends 26 miles from north 

 to south, and from 4 to 12, from east to west. The plants consist chiefly 

 of zamites, calamites, and equisetums, and these last are very commonly 

 met with in a vertical position more or less compressed perpendicularly. 

 It is clear that they grew in the places where they are now buried in 

 strata of hardened sand and mud. I found them maintaining their erect 

 attitude, at points many miles distant from others, in beds both abov^e 



