170 GEOLOGY. 



called Athyrus subtilita. It occurs in almost every layer of the 

 Carboniferous rocks and of many sizes. Among the Spirifers 

 the most abundant and beautiful is S. cameratus. Lamellibranchs 

 (two valved shells with gills in laminae on the sides) were repre- 

 sented in Nebraska during this age by at least forty species. The 

 Gasteropods (one valved, like snails. Belly creepers) were 

 abundant in individuals and species, not less than eighteen forms 

 having thus far been identified. Of chambered shells there was 

 one strait species (Orlhoceras cribrostim) and two coiled ones, 

 (Nautilus occidentalis and N. ponderosd). 



Of the five species of Crustaceans found fossil in these rocks 

 three are trilobites of the genus Phillipsia. 



Vertebrate life so far as is now known was represented here in 

 Carboniferous times principally by fishes, of which eight species 

 have been described by Orestes St. John. Many more have been 

 found which have not yet been identified. 



Climate. — The vegetable and animal life of the Carboniferous 

 Age indicates that its climate was not subject to extremes, at least 

 during the epochs when the rocks were deposited, whatever it may 

 have been during the transition intervals. It was neither intensely 

 hot nor cold. It was just such a climate as a constantly murky, 

 cloudy atmosphere, over semi-continental levels and flats would 

 naturally produce.* Tyndall has shown that a slight addition to 

 our atmosphere of carbonic dioxide would raise its mean tempera- 

 ture many degrees. If our atmosphere then, at that time, as many 

 geologists believe, contained the greater part of the coal deposits 

 of the globe in the form of carbonic dioxide gas, it would have 

 made it a huge hot house. This would account for the uniformly 

 warm temperature that then existed far into the arctic regions. 



Close of the Carboniferous Age. 

 In the eastern portion of the continent the Carboniferous Age 

 was evidently closed by the Appalachian revolution. This great 

 uplift was evidently continental in character, the level of the land 

 on each side being raised along with it. This was no sudden con- 

 vulsion. The Appalachians commenced to rise long before the 

 close of the age and during its progress a point was reached when 

 the old conditions were passed and new ones inaugurated. Vege- 



*The theory that the Coal Age was produced by a period of high eccentricity of the earth's, 

 orbit, during times similar to the subsequent glacial ages is best discussed in Croll's work o« 

 " Climate and Time." 



