264 THE GEOLOGICAL HISTORY OF PLANTS. 



sions. The Erian flora is comparatiyely poor, and its 

 types are in the main similar to those of the Carbonifer- 

 ous. Of these types a few only reappear in the middle 

 coal-formation under identical forms ; a great number ap- 

 pear under allied forms ; some altogether disappear. The 

 Erian flora of New Brunswick and Maine occurs side by 

 side with the Carboniferous of the same region ; so does 

 the Erian of New York and Pennsylvania with the Car- 

 boniferous of those States. Thus we have data for the 

 comparison of successiye floras in the same region. In 

 the Canadian region we have, indeed, in direct sequence, 

 the floras of the Upper Silurian, the Lower, Middle, and 

 Upper Erian, and the Lower, Middle, and Upper Car- 

 boniferous, all more or less distinct from each other, and 

 affording an admirable series for comparison in a region 

 whose geographical features are very broadly marked. 

 All these floras are composed in great part of similar 

 types, and probably do not indicate very dissimilar general 

 physical conditions, but they are separated from each 

 other by the great subsidences of the Corniferous lime- 

 stone and the Lower Carboniferous limestone, and by the 

 local but intense subterranean action which has altered 

 and disturbed the Erian beds toward the close of that 

 period. Still, these changes were not universal. The 

 Corniferous limestone is absent in Gaspe, and probably in 

 New Brunswick, where, consequently, the Erian flora 

 could continue undisturbed during that long period. 

 The Carboniferous limestone is absent from the slopes of 

 the Appalachians in Pennsylvania, where a retreat may 

 have been afforded to the Upper Erian and Lower Car- 

 boniferous floras. The disturbances at the close of the 

 Erian were limited to those eastern regions where the 

 great limestone-producing subsidences were unfelt, and, 

 on the other hand, are absent in Ohio, where the sub- 

 sidences and marine conditions were almost at a maxi- 

 mum. 



