PIPER FLORA OP THE STATE OF WASHINGTON. 37 



ORIGIN OF THE UPPER SONORAN FLORA. 



Some considerations regarding the origin of the Upper Sonoraii 

 species of the Washington flora seem to justify definite conchisions. 

 In the geographical history of the Columbia Plains, as above outlined, 

 there was found, during the existence of Lake John Day in Miocene- 

 Tertiary time, a rich flora of subtropical aspect. Subsequent to this 

 time and probably not much prior to the glacial epoch, occurred the 

 principal uplift of the Cascade Mountains, This undoubtedly 

 caused profound changes in climatic conditions, particularly to the 

 eastward of this range, accompanied by correspondingly great 

 changes in the flora. 



Succeeding the uplifting of the Cascade Mountains came the Gla- 

 cial period. During this period immense changes took place in 

 western Washington, but there is scarcely a trace of glacial work on 

 the plain of the Columbia. Nevertheless, in accord with the lower 

 temperatures there was probably a general southward, migration of 

 the plants, followed by their return on the retreat of the ice. 



Of the changes which have taken place in the flora since the John 

 Day period to the present time there is no record preserved. In 

 this enormous lapse of time — perhaps millions of years — a forest 

 composed of magnolias, elms, sycamores, etc., apparently much like 

 that of the Lower Mssissippi in the present day, has utterly vanished 

 and no near relatives remain, either in the same or adjacent territory. 



Inasmuch as somewhat similar subtropical forests existed in Mio- 

 cene time in western Washington, the causes which have led to their 

 extinction are more profound than such climatic changes as could be 

 occasioned by the Cascade uplift and are to be sought rather in the 

 general causes which have lowered the temperatures of the earth's 

 surface. 



There remains, then, but one source from which light may be 

 thrown on the present constitution of the flora — namely, its relation 

 to contiguous floras. 



Such considerations naturally focus first on the physical conditions 

 which most likely prevailed at the close of the last great geological 

 cataclysm, namely, the Glacial period. The most conspicuous result 

 of this period of cold is perhaps the large number of Arctic species 

 which occur on all the higher peaks of the Cascades or stranded on 

 isolated mountains, as the Olympics or the Blues — a fact which ap- 

 pears more striking in mountains farther to the southward. How- 

 ever slight the effect of the Glacial period may have been in eastern 

 Washington, it is quite certain that the temperature was such that no 

 plants adapted to Upper Sonoran conditions could survive. They 

 were either driven southward, as were the Arctic plants, or else per- 

 ished. Following the retreat of the ice, the areas that then became 



