PIPER FLORA OF THE STATE OF WASHINGTON. 69 



Raiiiic)-a sfricta. Holidat/o caiirina. 



Ru(lbcc/:i(i alpicuhi. ^phacnilcca loitf/isepala. 



Saxifruf/a apvtahi. Spiraea cincrascetis. 



Scnccio cliiieri. Valeria ihi eoJioiibiana. 



This extremely restricted range of so considerable a nuuil)er of 

 species in a mountain chain which would seem to offer no barrier to 

 their extension southward is perhaps du.e primarily to the fact that 

 the greater portion of the Cascade system from ]Mount Rainier south- 

 ward is volcanic. Indeed, the eruption of the igneous rocks not only 

 may have brought about tb.e isolation of tlie plants above mentioned, 

 but. through their preference for granitic soils, may have kept them 

 from spreading southward. This idea further obtains support in the 

 fact that there are some striking similarities between the flora of the 

 Mount Stuart region and tliat of the Klamath region in southwestern 

 Oregon, which is also largely granitic in character. 



This is exemplified by a number of species which do not occur in 

 the intermediate region. Among them are — 



Arahis siiffniteseeiis. Uoorehekia grecnei. 



BikukuUa unifiora. Kelloijt/ia galioidcs. 



Campanula seaineJIa. Ledum gland iilosum. 

 Chaenactis iieradcnsis. 



To these should i:)erhaps Ije added Cacalioi)sis nardosmia gJahrata 

 and C. nai'dosmia, Lulna lixjpoleued^ and L. hypoleuca calif oriiica. 



Facts to be adduced hereafter in connection with the make-up 

 of the flora of the Blue ^Mountains emphasize still more the above 

 conclusions. 



While a great portion of the plants of the Cascade and the Blue 

 mountains are identical, there are nevertheless many species wlioi^e 

 occurrence in the former mountains is so local that their recurrence 

 in the latter furnishes some strikingly peculiar facts. As before 

 stated, the central and southern portions of the Cascade system are 

 composed of recent volcanic rocks, while the northern portion and 

 the Siskiyou and other mountains of the Klamath region con- 

 tiguous to the southern end of the Cascades are made up of older 

 rocks, largely granite. This difference in geological structure seems 

 to be directly associated Avith the distribution of certain plants 

 here discussed. The facts of the distribution are, first, that there 

 are species in common between the northern Cascades and the 

 Klamath region which are absent in the intermediate portion of the 

 Cascades; second, that certain species occur only in the Blues and 

 the Klamath region or the northern Sierras; third, that others occur 

 only in the northern Cascades and the Blues; and finally some 

 species occur in all three regions, but not elsewhere. 



