COAL FORMATION. 45 



17. Marine shells are rare in coal strata, and are only found in 

 the subordinate limestone of Belgium and England; but at the 

 same time there were some species of unio and some small ento- 

 mostracans which indicate at least an afflux of fresh water to the 

 sea at the points where these particular deposits were made. 



Fig. 52. Tooth of Fig. 53. Tooth of Fig. 54. Tooth of 



Cestracion. Hybodon. true Shark. 



18. EXTENT OF THE COAL-MEASURES. It is evident that the 

 coal formation cannot be found except above the Cambrian, Silurian 

 arid Devonian strata, which were formed anteriorly to, or about the 

 time of these deposites. If it existed before that period, it must 

 be necessarily concealed by all the strata subsequently formed, and 

 searches have been extended below them at great expense for thi? 

 combustible. The consequence is, that the coal formation occupies a 

 small portion of the uncovered surface of the earth. All the depo- 

 sites known in France do not occupy more than one two-hundredth 

 part of the superficies of the territory. England and Belgium are 

 comparatively richer, for in the first the surface of the coal forma- 

 tion is equal to one-twentieth of the whole kingdom, and in the 

 second to one twenty-fourth. All the other States of Europe are 

 much poorer, and some, Sweden, Norway, Russia, Italy and Greece, 

 are almost entirely without this valuable formation. Bohemia is 

 the richest part of Germany in coal, although it does not produce 

 largely. The northern part of the Spanish peninsula seems to 

 contain considerable deposites of coal, and to participate, in this 

 respect, in the wealth of Western Europe. 



19. The coal-fields of the United States are numerous and ex- 

 tensive. Coal is found in Massachusetts, Rhode Island, Pennsyl- 

 vania, Maryland, Virginia, Ohio, Kentucky, Tennessee, Illinois, 

 Alabama, Mississippi, and Indiana; in a word, the coal formation 

 in the United States is greater than in any country or kingdom on 

 ihe face of the earth, and embraces every variety hitherto disco- 

 vered. 



20. The different layers, constituting the coal-measures, were 

 deposited horizontally at the bottom of the basins they occupy, but 

 they have not remained in this position ; at certain places they 



17. What does the existence of the genus unio in the coal-beds indicate 1 

 IS. What is the relative geological position of the coal-rneasures ? 

 19. In what pa-ts of the United States do we find coa-1? 



