MESOZOIC AND TERTIARY FLORAS ill 



tion of this type of vegetation was one of the most important and far- 

 reaching biologic events the world has known. For many years the 

 flora of the Dakota Group and kindred floras was the oldest angiosper- 

 mous flora known in this country, but as there are such a host of appar- 

 ently modern types present, it was presumed that they must have had 

 an ulterior period of development^ — and such proved to be the case. So 

 far as we now know this flora appears to have had its origin in eastern 

 or northeastern North America, in the Patapsco division of the Poto- 

 mac series. Although the great majority of the plants found in asso- 

 ciation in these beds, both as regards species and individuals, still 

 belonged to lower Mesozoic types, such as ferns, cycads, and conifers, 

 we find ancient if not really ancestral angiosperms, and many of 

 the same types are found in beds of approximately the same age (that 

 is Albian) at Circal in Portugal. Although we are here much nearer 

 . the origin of the angiosperms than was before known, we are proba- 

 bly still some distance from their actual point of origin, but just 

 where or when that was we do not, and may never know. 



No sooner were they fairly introduced, however, than they multi- 

 plied with astonishing rapidity and in the upper members of the Poto- 

 mac series — Raritan- — they had become dominant, the ferns and 

 cycads having mostly disappeared and the conifers having taken a 

 subordinate position. 



By the close of the Comanchan, or Lower Cretaceous, they had 

 spread as far north as Alaska and Greenland, and a large number 

 of modern genera were established. 



Climatic conditions during Comanchan. — The climate over this 

 vast area was certainly much milder than at the present time, for such 

 well-known plants as elms, oaks, maples, magnolias, and many others 

 were growing 72° N., in Greenland and nearly as far north in Alaska. 

 It was at least what we would now call warm temperate. 



Upper Cretaceous. — With the inauguration of the Upper Cretace- 

 ous the angiospermous flora was in full swing. 



On the Atlantic border we have the Magothy, which extended from 

 Maryland over New York, Long Island, and as far as Martha's Vine- 

 yard. The flora is a rich one, embracing about one hundred and fifty 

 species. 



In the interior, in approximately the same position, is the Dakota, 



