280 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 14, 



the oolitic period, from the character of the numerous marine shells 

 found in some of the associated strata. 



Upon the whole, therefore, I arrive at the same result as Prof. W. 

 B. Rogers, namely, that the coal-field of Eastern Virginia agrees in 

 age with the lower members of the oolitic or Jurassic group of Europe, 

 — a conclusion which is important, because these strata constitute at 

 present the only representatives of this group hitherto discovered in 

 the United States of North America. If future researches should 

 require any modification of this opinion, we may then expect that 

 the trias will be the group to which the American formation will be 

 referable. 



In conclusion I may observe, that I was much struck with the 

 general similarity of this more modern coal-field, and one of palseozoic 

 date near St. Etienne in France, which also rests on granitic rocks, 

 from the detritus of which its coarse grits and sandstone are composed. 

 In both these coal-fields upright Calamites abound ; fossil plants are 

 met with in both, almost to the exclusion of all other classes of 

 organic remains, shells especially being absent. The character of 

 the coal is similar, but in the richness and thickness of the seams, 

 the Virginian formation is pre-eminent. When we behold phgeno- 

 mena so identical repeated at an epoch so much more modern in the 

 earth's history, and at a time when a very distinct vegetation had 

 been estabhshed, we may derive from the fact a useful caution in re- 

 gard to many popular generalizations respecting a peculiar state of 

 the globe during the remoter of the two periods alluded to. Some 

 geologists, for example, have supposed an atmosphere densely charged 

 with carbonic acid to be necessary to explain the origin of coal — an 

 atmosphere unfit for the existence of air-breathing vertebrate animals ; 

 but this theory they will hardly be prepared to extend to so modern 

 an epoch as the oolitic or triassic. 



DESCRIPTION OF THE PLATES. 



Plate VIII. Figure of Dictyopyge macrura, size of nature, from the oolitic coal- 

 field of Eastern Virginia. This specimen was obtained from the 

 Blackheath mines in Chesterfield county, Virginia, None of the 

 parts here represented have been restored. See p. 275. 



b. The anal fin, which is not so perfect as in some other specimens from 



the same region. 



c. A faint impression of the pectoral fin. 



d. Part of the ventral fin and an impression of the remainder. 



e. Scale from the lateral line. 



f. Shape of the scales from immediately above and immediately below 

 the lateral line. See p. 276. 

 Plate IX. fig. 1. Posterior part of the same fish, D. macrura, from the same slab 

 of stone. 

 d. Ventral fin. 

 b. Anal fin. 

 fig. 2. Tetragonolepis : portion of a fossil fish from the same strata, 

 Blackheath, Virginia. See p. 277. 

 a. Scale of the lateral line. 



