1919] VESTAL— PHY TOGEOGRAPHV OF COLORADO 165 



granitic and sedimentary areas. This question has been discussed 

 by Ramaley (9), who found the two areas about the same in 

 floras (in the Poudre mountain-front area), with Cercocarpus 

 abundantly represented in the sandstone but not in granite, 

 Selaginella apparently absent from the sandstone, and lichens 

 infrequent there. Following a suggestion from Cowles, it appears 

 to the writer that differences in rate of erosion of the substratum 

 may explain the distribution of lichens, and perhaps Selaginella 

 also. The sandstones are rather soft in the Poudre area, and wear 

 away too rapidly for the lichens to establish themselves abundantly. 

 The Fountain sandstone is harder in the Boulder region than else- 

 where, and there at least it bears lichens almost as abundantly as 

 do the granites. Selaginella is frequent in the sedimentary rocks in 

 the Boulder area, as Ramaley has pointed out. The writer knows 

 of no plants which are restricted to either sedimentary or granitic 

 areas, the only observed differences being those of relative abun- 

 dance. The gulches, exposed slopes and crests, etc., of the sedimen- 

 tary area are quite comparable to similar topographic situations of 

 the granitic foothills, and have practically the same plant assem- 

 blages. 



The rocky upper slopes of the Fountain, the Dakota, and other 

 ridge-making strata, where they occur, lack soil except in crevices, 

 and are mostly bare, except where rock pines or pinyons, shrubs of 

 rocky situations {Cercocarpus, Ribes, Janiesia, etc.), and crevice 

 plants, including many xerophytic herbs, can obtain a foothold. 

 The west slope of hogbacks is blufflike, usually, and rocky, while 

 the east slope is less steep (depending on the local angle of dip) 

 and likely to be strewn with debris, as are the slopes of the harder 

 exposures of the valley, and these have shrubby or herbaceous 

 vegetation, sparse, and of species of rocky situations. The softer 

 shales occupying the bottom of the valley are usually deeply buried 

 by debris (of line soil with imbedded rock fragments of all sizes), 

 and support a grassland vegetation, which is luxuriant in the 

 rainier parts of the growing season and very dry the rest of the time. 

 A stream-bed in the bottom of the valley may be bordered by a 

 strip of mixed shrub, Crataegus, oak, or canyon forest; or if dry, 

 by scattered narrow-leaf cottonwoods and willows. Mesophytic 



