288 THE PLANT WORLD. 



miles long, one and one half miles wide, 1800 feet deep on the 

 Washington side, and 2500 feet deep on the Oregon side. The 

 mesa, or table-land, is composed largely of loess, a decomposed 

 lava dust ; the contour of rolling hills and dales is formed by 

 this wind-drift carried by and in the direction of the prevailing 

 winds. This drift is from 90 to 150 feet thick where attempts 

 have been made to reach water by digging. After the surface 

 moisture has been penetrated, none other is found. In a few 

 basins temporary ponds are formed after a winter rain. The 

 soil has an alkaline impregnation, as is indicated by Chenopodium 

 album, which grows in these depressions. The winds, in giving 

 a roundness to the contour, often dig out " sand-blows," which 

 are the analogue of " pot-holes " ; in other places " dunes " may 

 travel over a ridge to disappear on the lee side, all of which 

 seems to have a marked effect on the character of much of the 

 life of the region. 



The annual rainfall varies from seven to eleven inches ; one 

 third of this is in the form of snow, while a small portion comes 

 from the rime and frost which accumulates on every available 

 object during the prevalence of a severe northeasterly wind ac- 

 companied by its usually attendant fog which, freezing, forms 

 acicular points of hoar-frost even to two or three inch crystals. 

 These " Walla Walla winds " are cold and penetrating, in winter 

 bringing dust storm, snow storm or blizzard. During many 

 days this gorge is filled with fog, while the mesa is clear and 

 sunny. The current of this river of fog is determined by the 

 wind. This body of dense fog hovering over the body of water 

 keeps up the general temperature of the gorge, and this may in 

 part account for the variety of rock-loving lichens therein. 



The plant-life of this region occurs in three distinct vertical 

 zones: (a) The mesa and "breaks," (b) face of cliff's and talus, 

 (c) the margin of the rise, or littoral. There are no bottom or 

 alluvial lands. The following observations thereon extend over 

 a period of nearly six months, from October 17, 1905, to April 

 10, 1906. 



On several basaltic reefs extending well out into the river 

 and but six or eight feet above water at low stage, was found 



