MESOZOIC AND TERTIARY FLORAS 205 



descendants of which are now found in southern lands, all point to a 

 moist, warm, probably subtropical climate, though in late Jurassic 

 time the presence of well-defined rings in the tree trunks of species 

 found in northern areas — King Karl's Land, Spitzbergen, etc. — show 

 that there were beginning to be sharply marked seasons. 



Wealden. — Immediately above what by common consent is 

 regarded as the top of the Jurassic, is a series of fresh-water plant- 

 bearing beds that are of quite wide extent in this country, though 

 different names have been applied in the different areas. Thus the 

 lower Potomac of the eastern United States (including the Patuxent 

 and probably Arundel), the Glen Rose beds of the Trinity division 

 of Texas, the Lakota and Cloverly of Dakota and Wyoming, the 

 Kootenae of Alberta and adjacent Montana and extending into the 

 Bighorn Basin of Wyoming, and the Shasta of California, and Kome 

 of Greenland are practically equivalent in age, and correspond most 

 closely in age with the Wealden of the Old World, which is considered 

 to be a fluviatile or lacustrine condition of the lower Neocomian, the 

 lowest member of the Cretaceous. The flora is a comparatively rich 

 one, aggregating between two hundred to three hundred species, and 

 is composed of ferns and conifers with a fair sprinkling of cycads 

 Equisetaceae, ginkgos, etc. It shows a considerable agreement with 

 the Jurassic, a number of species being common to the two, but on the 

 whole its affinity is rather with the Cretaceous. 



Cretaceous. — Up to the present point in the geological column the 

 most characteristic and dominant feature of the modern flora — namely 

 the angiosperms — has been absent. In many ways the introduc- 

 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, 



