. Results of a biological survey of mount Shasta, California. ). Herean avalanche of unusual size must have shot dowri the higher slopesuntil it reached the upper edge of the continuous forest of Shasta firs,where, instead of stopi)ing, it cut a broad swath tliiougli the hugetrees, tearing them up by the roots or snaiq)ing tliem off and carrying OCT., 1899. TIMHERLINE. 27 them oil over an almost level tract with such resistless force that thefew now left standing arc deeply scarred at a height of 3 0 to 15 feetabove the ground, showing where they were struck by other trees inpassing over the d
. Results of a biological survey of mount Shasta, California. ). Herean avalanche of unusual size must have shot dowri the higher slopesuntil it reached the upper edge of the continuous forest of Shasta firs,where, instead of stopi)ing, it cut a broad swath tliiougli the hugetrees, tearing them up by the roots or snaiq)ing tliem off and carrying OCT., 1899. TIMHERLINE. 27 them oil over an almost level tract with such resistless force that thefew now left standing arc deeply scarred at a height of 3 0 to 15 feetabove the ground, showing where they were struck by other trees inpassing over the deep snow. Hundreds of huge trunks To to 100 feetlong and .> or 4 feet in diameter are strewn in desolate confusion overthe broad area tlnit marks the place where this terrific avalancheslowed up and hnally stopped. The accompanying illustration showsa part of this area, and also the gate cut by the avalanche when itstruck the upper edge of the Fig. 13.—Track of avalanche iii\ adiuii lll^e^^t nt Sliasta liis uii Cold Creek, side of Shasta. TIMBERLINE. Timbeiiine is the upj)er or boreal limit of tree growth, as determinedby temperature. It varies somewhat according to the particular speciesof tree, for even Hudsonian species differ in the degree of cold theyare able to endure. Thus in the northern Cascades where the alpinehemlock and alpine fir are the dominant timberline trees, the fir pushesup to higher altitudes than the hemlock. So on Shasta, where thealpine fir is replaced by the white-bark pine, the latter is the true tim-berline tree and always attains higher elevations than the hemlock. Theoretically, nothing is easier than tracing timberline on a moun-tain \vhose upper slopes are bare or dotted with alpine fiowers andwhose middle slopes sui)port a continuous forest. Yet on Shasta, andon most high mountains, it is exceedingly difiicult to fix the boundaryof timberline or indicate its exact position on a map. O
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