42, THE Ul\fY7.RSITY OF OKLAHOMA 



to ou- knowledge of the subject. The Trinity sands of Oklahoma 

 and "^rexas which are of the same age as the formations named, 

 contain leaf fragments, but no determinable species have been ob- 

 tained. J\'Iany species of Plataniis are found among these var- 

 ious beds, and so far as I have been able to learn the largest 

 Plantanus leaf, if indeed not the largest fossil dicotyledonous leaf 

 ever recorded, over 18 inches in diameter, came from the Koon- 

 ttnai formation of Northern Washington. 



The greatest single leaf-bearing formation in the v/orld is 

 probably the Dakota Cretaceous sandstone. This formation under- 

 lies all the central and northern Great Plains, outcropping along 

 the Rocky Mountains front, the Black Hills, and along a line 

 extending from, northern New Mexico, across western Oklahoma, 

 central Kansas, eastern Nebraska, Iowa, and South Dakota, far 

 into Minnesota. Hundreds of exposures of the dark brown 

 Dakota santstonc throughout this area have yielded tens of thou- 

 sands of dicfltylcdoiis, including types of not only a great part of 

 the genera living in this country today, but also of a number of 

 forms not now indigenous to this part of the world, such as 

 m.agnolia, fig, eucalyptus, gingko, and the Sequoya, or big trees 

 of California. 



Among the genera found in the Dakota, one of the most 

 abundant is Platanus. The leaves were not so large as in Com- 

 anchean times, but the forms were more varied giving rise to 

 a greater number of species. All the leaves are well formed, 

 and symmetrical, indicating hardy and vigorous trees. Rocks of 

 the same age as the Dakota, in Greenland, Spitzbergen, and on 

 the Pacific Coast of the United States have yielded many species 

 of Platanus. This age represented the culmination of the genus. 



During Tertiary times the Platanus began to decline. The 

 species became fewer, and the types of leaves were less vigorous, 

 showing a decadence in the genus. The Tertiary of the High 

 Plains of Beaver, Cimmarfon, and Texas counties, Oklahoma, have 

 3nelded a few fossil leaves, among Platanus, but not sufficient 

 for specific indentifications. 



In Quaternary times the Platanus became rare. The genus 

 was declining rapidly. The leaf-beds found in the glacial till, 

 and among inter-glacial deposits of Europe and America, show 

 great number of species of such forms as oak, beech, birch, willow, 

 liquid amber, populus, maple, and elm, and but some scant half 

 dozen of the Platanus. The genus was slowly yielding to the 

 inevitable. Having reached its clumination during the middle 



