MESOZOIC AND TERTIARY FLORAS 107 



ferns are of large size, indicating luxuriant growth, while Equisetum 

 stems 4 to 5 inches in diameter are only approached by a single 

 living South American species. The cycads are not more depau- 

 perate than those of subsequent horizons, nor do they compare 

 unfavorably with the living representatives. 



The complete, or nearly complete absence of rings in the tree 

 trunks indicate that there were no, or but slight, seasonal changes 

 due to alternations of hot and cold, or wet and dry periods. The 

 accumulations of coal — in the Virginia area aggregating 30 to 40 

 feet in thickness — indicate long-continued swamp or marsh condi- 

 tions, while the presence of ferns, some of them tree-ferns, indicate 

 oil the whole a moist, warm, probably at least sub-tropical climate. 



Jurassic— Coming, now, to the Jurassic, we find in the lower 

 portion indications of a continuation of conditions which obtained in 

 the upper portions of the Trias. The distinctive Paleozoic elements 

 had finally disappeared, and the Mesozoic life-forms were in full 

 swing, expanding in the middle and upper parts of the period into 

 the abundant and widespread flora as we know it. In fact the 

 relative . uniformity and wide extension of the Middle and Upper 

 Jurassic flora is one of the most interesting and impressive exhibits 

 that we have. (See map showing approximate distribution of Triassic 

 and Jurassic flora.) 



There is no paleobotanical evidence indicating the presence of 

 the Jurassic in Eastern North America. In the western interior 

 Jurassic plant-bearing beds occur in the Black Hills, South Dakota, 

 and the Freezeout Hills, Carbon County, Wyoming. We then pass 

 to the Pacific coast, where we have a fine flora near Oroville, Cali- 

 fornia; also northward in Trinity and Tehama counties, California, 

 and Douglas and Curry counties, Oregon. 



The following is an outline of the world distribution of the flora: 



Alaska Copper River District 



Cook Inlet 



Herendeen Bay 



Cape Lisbume 

 England Yorkshire 



France Mamers — northwestern portion 



Germany Franco-Swabian area 



Northwestern area 



