90 TREE ANCESTORS 



or trembling with the characteristic motion and sound that gives 

 them their vernacular names. This gave rise to the tradition that 

 aspen was the wood of the cross — its leaves have quivered ever 

 since. Other interesting poplars are the so-called cottonwoods of 

 the West, where they are almost the only native trees in the river 

 valleys of the prairie country, ranging from Assiniboia to New 

 Mexico. The cottonwood has narrower leaves than the rest of 

 the poplars and in the commonest species it approaches a willow 

 leaf in appearance. 



Neither ^villow nor poplar timber can compete with larger and 

 stronger woods such as pine or spruce or with more durable woods 

 such as oak, cedar, and chestnut. Pulpwood, excelsior, and fuel 

 are their largest uses although for bam floors, boxboard veneer, 

 spools, matches, etc., their qualities of softness, lightness, ease of 

 working and lack of splintering, render them valuable. 



The geological history of the poplars is most interesting although 

 somewhat obscure. About 125 fossil forms have been described, 

 in addition to the still existing species that are found fossil in the 

 Pleistocene deposits but a number of these are of questionable 

 identity. The oldest known were the contemporaries of the dino- 

 saurs of the closing days of the Lower Cretaceous. One small 

 leafed form is found at this early day in the Potomac River valley 

 and the other, which was for a long time the oldest known dicotyle- 

 don, comes from the Kome beds of western Greenland and was 

 named Populus primaeva by Heer, its describer. 



These first poplars are rare forms but their geographical separa- 

 tion gives us a hint that their abundance was greater in those early 

 days than the records show, and this is also indicated by the abun- 

 dance and wide distribution of poplars during the Upper Cretaceous, 

 from which about 30 species have been described. They are much 

 less abundant than the willows in the Upper Cretaceous of Europe 

 but, unlike the willows, they are common in Greenland, and they 

 are exceedingly ubiquitous in North America at this time, especially . 

 in the West where they appear to have been very common along 

 the borders of the Upper Cretaceous sea that submerged so much 

 of the then low western country. In addition to the American^ 



