28 NOKTH AMERICAN FAUNA. [no. 16. 



possible to follow the uppermost trees wherever they may lead, but a 

 uiaj) showiii<.i- sudi a route WKuld resemble a saw from which alternate 

 teeth had beeu removed, the remaining teeth indicating the way the 

 dwarf trees push up on the summits of ridges, the broad spaces 

 between the teeth, the treeless gaps, usually the intervening valleys 

 or basins. Trees always occur at some i)oint in tlic bottoms of these 

 valleys, and usually extend completely across them, but at an altitude 

 a thousand feet or more lower than that reached on the ridges, and 

 there is a material ditlerence in the trees themselves. If of the same 

 species, those in the valleys are much larger and taller; if of other 

 species, as is frequently tlie case, they belong to the ui)per i)art of the 

 belt below — the middle forest belt. On Shasta, the trees that push up 

 highest on the ridges are always the dwarf wliite-baik i)ines, while 

 as a rule those that bridge the intervening valleys below are full- 

 grown alpine hemlocks or Shasta iirs, the upper limit of which must 

 not be mistaken for timberline. The difticulty lies in determining 

 what ought to be considered true timberline, and the reason why 

 in the absence of obvious barriers the white-bark pines do not lill more 

 than a third or a fourth of the belt to which they proi)erly belong. 

 If a mountain could be found whose upper slopes form a true cone 

 instead of a series of alternating ridges and valleys, so that suc- 

 cessive transverse sections would be circular in outline, instead of 

 irregularly scalloped, it is probable that timberline would form almost 

 a true circle around the peak, rising a little on the southwest and 

 dipping down a little on the northeast. But in the absence of such 

 ideal conditions, actual visible timberline is usually confined to the bor- 

 ders of the tongues of dwarf trees tliat occupy the summits of the radi- 

 ating ridges (pi. iv). The explanation of the absence of trees from the 

 intervening valleys is not always easily found; still, if the valleys are 

 studied with reference to the details of their several slope exi)osures 

 and other local conditions, the position of the hypothetical timberline, 

 in most cases, will be obvious. Let us take, for instance, oiu3 of the 

 numerous glacial basins on the south side of Shasta, bordered on each 

 side by lofty ridges which are capped by tongues of white-bark pines. 

 The bottom of the valley, whenever its axial slope is steep enough to 

 be regularly swept by avalan(;hes, can not, of course, contain trees. The 

 broad basin slope of the ridge on the west faces east and is in its own 

 shadow in the afternoon; as a consequence it is too cold ibr trees, but 

 is well sprinkled with alpine plants. Its summit is covered with dwarf 

 white-bark ]»incs, which come up from the other side and end abruptly 

 along its eastern crest. The cold eastern slope is, in its /one ])osition, 

 actually above timberline, althcmgh the tongue of dwaif trees along 

 its crest may stretch up a thousand feet above the lowest al]nne 

 idants. 



On the op]>((sitc or eastern side of the basin the slope faces west or 

 southwest, and receives the warm ra\s of the afternoon sun. The 



