i66 BOTANICAL GAZETTE [September 



ravines developing in the sedimentary area support mixed-shrub, 

 woodland, or mesophytic herbaceous growths, as in the granitic 

 foothills. Local meadows (mesophytic grassland) are found on 

 slopes where seepage or a high water table moistens a deep soil for 

 at least part of the growing season. 



In places the sedimentary rocks have been worn down more than 

 is common, so that they are mostly or in part reduced to a general 

 grade, above which the more resistant layers rise locally. This 

 is the condition in the valleys of some of the larger streams from 

 the foothills, and is seen at Platte Canyon, partially at Bear Creek 

 (Morrison), and also in Perry Park (fig. 4) and the Garden of the 

 Gods. The floor of this graded surface, especially in the Fountain 

 exposures, is likely to be covered very thinly with small angular 

 fragments, loose or compacted. The vegetation, as well as the 

 soil, is very like that of gravelly floors in the foothills, being a 

 variant of the primitive grassland association, with scattered rosette 

 or mat plants, Boiiteloua hirsuta, etc. 



The climatic transition in the zone of upturned sedimentary 

 strata is rapid. At Boulder and elsewhere dense cloud-banks 

 have frequently been seen to descend to or just beneath the Foun- 

 tain crags without continuing outward and downward to the 

 plains (figs. 6, 7). The outer granitic hills and upper sedimentary 

 slopes receive greater and more frequent precipitation than the 

 lower slopes and adjacent mesas and plains; it may rain slightly 

 below while it snows considerably above (cf. fig. 12); the outer 

 and lower slopes are more exposed to wind, less cloudy, and in 

 places less shaded from the afternoon sun by the higher granitic 

 hills than the inner valleys and upper slopes. No exact data are 

 available for this sudden climatic transition. Where the outcrop 

 of sedimentary ridges and valleys is wide, as in the northern 

 mountain-front region, the outer hogbacks are severely exposed 

 to sun and wind, as in the open plains. Their coarse rocky soil 

 favors woody plants; the xQroY>h.yt\QCercocarpus shrub assemblage 

 is here more extensively developed than anywhere else. 



Mesas and graded slopes of the debris-apron (fig. 8). — 

 The general character of the graded slopes and their mesa-fragments 

 has already been suggested. The mesas are of varying ages and 



