INTRODUCTION. 
65 
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 palaeozoic 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 period. 
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 time 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. 
[ Palaeontology III.] 
9 
