THE "slide rock" SUCCESSION 117 



and yellow pine. In some places their sides are occupied by 

 mature larch. The rest of these ridges are open and grassy 

 and show evidence of seasonal drouth. Among the grasses 

 (mostly Agropyron) may be found Sieversia ciliata, Zygadenus 

 venenosus, Helnginella densa and Arctostaphylos TJva-ursi. Such 

 conditions are impossible for the larch which seeks the moister 

 situations. 



The depressions between the vertical ridges are occupied 

 by lesser vegetation in various stages of development. The slide- 

 rock is thoroughly covered Avith encrusting lichens. When suf- 

 ficient hold is provided for mosses these appear, along with the 

 more foliaceous type of lichens. 



The first of the mosses to take hold on the exposed rock 

 surface is Grimmia p'tilvinata. Its dark cushion-like growths 

 accumulate sand, dust and vegetable matter, and ultimately fur- 

 nish a foothold for other small plants. 



Places are found in which Dicranum congestum fills in 

 between the rock fragments or wholly overgrows them. Woodsia 

 scopidina is evident and an occasional Sedum stenopctalum and 

 Heuchera glabella. At certain points shrubs become established 

 and among these maj^ be observed Pkiladelphus Lewisii, Amelan- 

 chier alnifolia, Opulaster malvaceus, Ribes cereum, R. saximoiv- 

 tanum, Rtdnis parviflorus, R. strigosus and others. It is inter- 

 esting to note the extent to which some of the more favored slopes 

 have become forested. Considerable areas have become covered 

 with a stand of vigorous Douglas spruce about 30 years of age. 

 The trees number as high as 3000 to the acre and so dense is the 

 canopy of their foliage that but few plants and these the most 

 tolerant find sufficient light to support life. Digging away the 

 thin cover of leaf mould one soon uncovers just such rock frag- 

 ments as form the usual talus. 



Elsewhere on the slope the stand may be practically pure 

 young larch, or with some mingling of Douglas spruce. The 

 extent to which the larch is dependent upon the ground water 

 is beautifully exemplified in places on this slope. Long drifts 

 of snow may be seen at the end of March ar early in April lying 

 in the shallow drainage channels and rising out of the drifts 

 are thickets of larch of various ages from small saplings to large 



