of North America with those of Europe. 363 
capar and Calymene Blumenbachii, of the blue limestone, which 
Tepresent the period of the lower Silurian. Near Galena upon 
‘the upper Mississippi, the line of separation of the two divisions 
1s yet more obscure, and there the distribution of fossils is not in 
harmony with the mineralogical differences. The blue limestone 
the magnesian limestone are still found, but they do not cor- 
‘Tespond exactly, as in the state of Ohio, with the two great 
‘epochs of the Silurian system. ‘The magnesian limestone thick- 
“ens at the expense of the blue limestone, and at its base it con- 
‘tains for the most part the fossils of the latter. 
With our limitations, the lower epoch of the Silurian system 
a very great extension in North America. Commencing at 
the island Anticosti, it follows the borders of the St. Lawrence, 
passes to the north of the Lakes Ontario, Huron, Michigan, and 
Teaches the Mississippi at Dubuque, Iowa. To the south, it fol- 
‘Waters of the sea, during the deposition of the more recent beds. 
* This very natural division has been recognized in the Paleontology of New 
York, vol. i, and will be more fully shown in vol. ii, where the intimate relations 
‘of the Medina i ps are clearly pointed out.—J. H. 
Szcoxp Serrzs, Vol. V, No. 15.—May, 1848. 47 
