GENERAL OBSERVATIONS. 15 



lington and Keokuk limestones that are more especially characterizedjby a 

 great preponderance of these forms. Although some crinoidal remains exist 

 in those Subcarboniferous strata that have been discovered in the Rocky 

 Mountain region, in none of them have they been found in so great profu- 

 sion as they exist in the two formations in the Mississippi Valley that have 

 just been named. In this respect, the collections from those western locali- 

 ties accord more nearly in faunal characteristics A'vith the other three forma- 

 tions of the Mississippi Valley series. 



It could not be expected that collections of Subcarboniferous fossils so 

 meager as those made by the expedition are should afford any very com- 

 plete indication of the relative prevalence in that region of the different 

 forms of marine life of the period ; but it may be well to note that they con- 

 tain no remains of fishes, no Articulates, and no Cephalopods, arthi-opoma- 

 tous Brachiopods being the prevailing forms. Such deficiencies as those 

 noticed are, however, not uncommon in much larger collections from typical 

 Subcarboniferous strata. 



CARBONIFEROUS PERIOD. 



The accession, in the Carboniferous age, of the conditions necessary 

 to the formation of coal was not simultaneous in all those parts of North 

 America over which deposits of that age were made ; nor were these con- 

 ditions sooner or later co-extensive with all parts of the area in which those 

 deposits exist, not even with those of the Carboniferous, or so-called Coal- 

 Measure, period. It is also known that these conditions, even during the 

 period of their greatest prevalence, occasionally ceased by shifting elsewhere, 

 and were resumed again ; alternating thus with conditions similar to those 

 that prevailed at the beginning of the age, before the first coal-deposits were 

 formed. In what are now portions of Pennsylvania, Virginia, Kentucky, 

 and Indiana, these coal-making conditions began before the close of the 

 Subcarboniferous period. Although then- prevalence became general during 

 the deposition, in the eastern half of North America, of the strata of the 

 first and second epochs of the Carboniferous period, especially the first, the 

 strata of the third epoch of the last-named period are usually as destitute 

 of coal as those of the Subcarboniferous period are. 



Indeed, the conditions that prevailed during the Upper Coal-Measure 



