16 GENERAL OBSERVATIONS. 



epocji were essentially a repetition of those which prevailed during- the 

 Subcarboniferous period. Bearing this in mind, it is easy to understand 

 that those species which are found in both Subcarboniferous and Carbonif- 

 erous strata may have reached the latter position merely by continuous 

 geographical distribution during the progress of the two periods. Geolo- 

 gists generally divide the strata of the Carboniferous period, in the eastern 

 part of North America, into Upper and Lower Coal-Measures ; but, in Iowa 

 and Missouri, they are more or less naturally divided into three fonnations, 

 as before indicated. Westward from tliose States, the strata of the Carbo- 

 niferous period have not been separated into corresponding epochal divis- 

 ions, and are perhaps not capable of such separation. In the Kocky 

 Mountain region, the strata of this period are widely distributed, and attain 

 a very great thickness compared with that of those in the Mississippi Valley. 

 Those far western Carboniferous strata probably represent in the aggregate 

 the whole Carboniferous period, but in their general lithological and pale- 

 ontological characters they are all much more nearly like the strata of the 

 Upper Coal-Measures as developed in Iowa, Missouri, and Nebraska than 

 they are like the Middle and Lower Coal-Measures. So far as known to 

 me, no considerable deposit of coal has been found in any strata of the 

 Carboniferous age in the Eocky Mountain region. The accession and 

 cessation of the physical conditions necessarily attendant upon the forma- 

 tion of coal seem to have constituted the principal means of marking, in 

 the accumulating strata, the epochal divisions of the period in Eastern 

 North Amei'ica. Those conditions of coal-making not having prevailed in 

 the Far West, and, so far as known, the physical changes that occm-red there 

 during the period not being coincident with those farther eastward, its 

 epochs were not there marked off in the same manner. 



The collections contain a greater number of species from strata of this 

 period than from those of any other ; and they were also found more 

 abundant than those of any one of the older periods that have just been 

 noticed. 



Of the sixty-two species that have been described or noticed in this 

 report and assigned to this period, one is a Rhizopod ; six, Actinozoa ; two, 

 Echinodermata ; tlu'ee, Polyzoa ; twenty-nine, arthropomatous Brachiopoda ; 



