INTRODUCTION. Btf 



panied by evidences of their relations to the true Coal measures. At the 

 same time, several writers have cited the occurrence of true Coal measures 

 in this region of country. 



In any event, however, it seems quite probable that the coal, and the 

 shales and sandstones which usually accompany it, have so far thinned out 

 as to be of comparatively little importance in the series. Whether this 

 conclusion prove wholly correct or otherwise, in regard to this part of the 

 country, it is evident that when we find the calcareous accumulations so 

 far preponderating over all the other materials, such must be the ultimate 

 result, unless some other source of land-derived materials shall be found. 

 We are led to anticipate this latter condition from the occurrence, in the 

 central and western parts of the continent, of large accumulations of the 

 older palteozoic sediments which are evidently derived from a different 

 source, and have had a different direction in their distribution. It is possi- 

 ble, therefore, that there may have been other sources for the derivation of 

 terrestrial vegetation and shore-derived materials during the Coal periods 



Whatever may have been the ancient condition of the central part of 

 this continent, it is clear, from what we know of the great extent of the 

 limestone of this age, that the ocean must have held entire dominion over 

 this region for a long period, even after the final deposition of these lime- 

 stones*. 



Thus the Carboniferous period, so designated from its characteristic 

 coal-fields in the region where the formation was first explored and 

 studied, cannot, from our present knowledge, be considered universally the 

 period of vegetation. Had our observations begun in the Rocky mountains, 

 and extended over the area from the Northern ocean to the Gulf of Mexico, 

 we should nowhere have found reasons to designate this as pre-eminently 

 a plant-bearing period. On the contrary, it is everywhere in that region 

 characterized by its fossils of marine origin, and in truth is apparently one 

 of the most, if not the most, extensive marine formation upon our conti- 



• At the lime of this writing, the series of strata between the limestones of the age of the Coal mea- 

 sures and the beginning of the Cretaceous period, have not yet been sufficiently explored to speak with 

 certainty of the conditions which existed during this interval. 



[ PALiEONTOLOGY III.] 9 



