THE ARKANSAS COAL MEASURES. 201 



the Urals, probably in Sumatra, and in Arkansas. The genus 

 Pronorites, while found in western Europe, is rare in it, and is much 

 more common in the Pacific region. Pronorites is found in the 

 Artinsk region and in Arkansas, while the ammonite genus 

 Medlicottia, the direct descendant of Pronorites, is found in the 

 Permo-Carboniferous strata of Sicily, the Urals, the Salt Range, 

 and Texas. 



It is impossible to suppose that the same genus and species 

 originated at different localities, and since we have both ancestors 

 and descendants in places so widely separated, we can only sup- 

 pose that there was free interchange of life between those places 

 at that time, or in other words an open sea, on the borders of 

 which these fossiliferous deposits were laid down, and through 

 which the cephalopods and other marine animals could migrate. 



Replacetnejit of Limestones by coal-bearing formatio?is in zvestern 

 Europe. — On tracing the Upper Carboniferous deposits of the Ural 

 region towards the west, we find the limestones thinning out, and 

 the Coal Measures and Culm formations taking their places ; we 

 find also that the transgression of marine on terrestrial deposits 

 takes place from the east, just the reverse of what is seen in 

 America. 



La?id areas in the West. — It is not thought that the Pacific 

 Carboniferous sea was an unbroken expanse of water in western 

 America ; on the contrary there are many evidences of large iso- 

 lated land areas and archipelagos. 



Dr. Joseph Le Conte' has argued that the Basin Range, during 

 much of Paleozoic and Mesozoic time, was a continent, off the 

 western shores of which the sediments that afterwards became the 

 Sierra Nevada and Coast Range were laid down. Clarence King" 

 thought that the great thickness of Paleozoic littoral deposits in 

 the Great Basin region proved the existence of a large body of 

 land further west ; he thought that the eastern shore of this con- 

 tinent was in Nevada, and east of this stretched the Carboniferous 

 sea, which covered all but the island chain of the Rocky moun- 



' American Journal of Science, III., Vol. i6, p. io8. 

 ^U. S. Geol. Explor. Fortieth Parallel, Vol. I., p. 534. 



