278 ■ THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



angiosperms abounded and predominated, several having living genera, but 

 witli them raan}^ of the plants found lower down in the formation still 

 existed. The beds at Brooke, White House Bluff, and Baltimore are types 

 of this portion of the formation 



It is a noteworthy fact that many of the fossils that are nearly allied 

 to living genera are compi-ehensive types, showing perhaps ancestral forms 

 of living types. The same may be said of the conifers that are associated 

 with these angiosperms, and winch show affinities with living genera of con- 

 ifers. They are not, then, either the conifers or the associated angiosperms, 

 so recent in type as a hasty inspection might indicate. When in any 

 series of beds at a given localit}^ the fossils ai*e of the character of those 

 found at Fredericksbui-g and at Dutch Gap, we may conclude that the 

 abseiice of angiosperms is explainable by the removal by erosion of the 

 upper strata, which contain them in large numbers. 



Another fact that should be noted as probably indicating the really 

 great antiquity of the most modern-looking angiosperms is the great rarity 

 of individuals in most of these, and it is the most recent types that show 

 the fewest individuals. This probably means that the type is of recent 

 introduction. This feature should, however, be accepted with caution as a 

 basis for reasoning concerning the scarcity or abundance of forms, since 

 we know nothing concerning tlie conditions which favored or opposed the 

 preservation of given plants. Apparent relative abundance or rarity may 

 be determined solely by the conditions controlling fossilization. Then, too, 

 the amount of material obtained from a given locality has, of course, much 

 influence in fixing the apparent abundance of the plants found only at 

 that spot. These remarks apply with special force to the Potomac flora, 

 which shows such a remarkable localization of the plants. It will be, 

 perhaps, proper to state in this connection that a very large amount 

 of material was obtained at the entrance to Trent's Reach; fishing hut 

 above Dutch Gap Canal ; Fredericksburg ; the two Brooke localities ; a 

 smaller amount, but still large, at White House Bluff, the two Balti- 

 nore localities, and th« two Potomac Run localities, and very little from 

 the others. 



