96 SIXTEENTH REPOET ON THE CABINET OF NAT. HISTORY. 



accumulation of the Coal measures. The series of fifteen or eigh- 

 teen thousand feet in thickness on the northeast gradually dimi- 

 nishes, till in Pennsylvania it is not more than one-fourth or one- 

 third as great ; and in the Mississippi valley it has not as many 

 hundreds, as in Nova-Scotia it has thousands, of feet. 



Still farther to the west and southwest it has lost its sedimen- 

 tary character, giving place to calcareous shales, marls and lime- 

 stones ; and the magnificent flora, which marked every stage of 

 its accumulation in the eastern and central regions, has entirely 

 disappeared. It is here no longer the great period of vegetation ; 

 and its identification over hundreds, and even thousands of miles, 

 is dependent upon the remains of a few marine animals. Fortu- 

 nately we have a few otherwise insignificant marine beds in the 

 midst of the sedimentary deposits of the Coal measures in Ohio 

 and Virginia ; and but for the continuance of the fossils of these 

 beds in the increasing calcareous accumulations in the far west 

 and southwest, we should there have no means of determining the 

 age and extension of this, elsewhere the greatest sedimentary and 

 plant-bearing formation in the geological history of the globe. 



In the same manner, the land-derived materials of the Devonian 

 period gradually diminish in a southwesterly direction, and finally 

 give place to other accumulations, ceasing to be marked by the 

 characteristic flora; while the littoral fauna gradually gives out, 

 or is replaced by another adapted to the changed conditions. 



Somewhere in this wide extent, we shall probably find that the 

 gaps, which elsewhere exist between the Devonian and Carboni- 

 ferous strata, are filled by beds of passage, or those beds which, 

 completing the series, leave no strong lines of demarcation be- 

 tween groups or systems. 



The foregoing observations have been suggested by the" perusal of the 

 very important paper of Professor Dawson on the Flora of the Devonian 

 Period. Having never proposed to make a special study of the fossil plants, 

 I have collected those which came in my way ; intending at the proper time 

 to submit them to some person engaged in these investigations, who, with 

 more extensive collections for comparison, could bring out more satisfactory 

 results, than could possibly be done with the slender materials furnished 

 by the rocks of New-York. 



The previous investigations of Professor Dawson in the Northeastern 

 Devonian Flora made it very desirable to place in his hands the material 

 derived from the rocks of New- York, in order that unity might be given 

 to the entire subject ; and I had no difficulty in obtaining the consent of 

 the Regents of the University to such a disposition of the collections. 



