118 FOREST DISTEIBrTIOX 



trees. One of these belts is not more than 15 feet wide and 

 extends doTSTi the slope for some hundreds of yards. 



The same history of the slid--: rock has been observed in 

 many places and the character o' its vegetation man be corre- 

 lated with that prevailing iv the locality and with the direction 

 of exposure. The identity of its vegetation varies with the alti- 

 tude and geographic position but the course of events is usually 

 the same, the lichen crust, the mosses of the Grimmia type, the 

 herbs, the shrubby vegetation and eventually the forest or the 

 prairie, according to exposure and climatic conditions. 



The Western Yallevs. 



"Whitford has described the mesophytie type of forest 

 of the Flathead Valley as characterized mainly by the pres- 

 ence of Douglas spruce and western larch but including also 

 Pinits contarta, P. mGniicola. Abies grandis, Thuja plicata 

 and Pirra Engelmannii. It would seem, however, as indic- 

 ative of the purely mesophytie tj-pe, that the species which 

 should be taken as characteristic are Finns montieola, Abi^s 

 grandis, Tsuga heferophijlla, and Tlvuja plicata. These species 

 are almost invariably, associated with one another in this region 

 and forests containing them are not only of restricted distri- 

 bution in ifontana, but also are marked by other more or less 

 distinctive species among the lesser forest plants. This group 

 will be considered later. 



While the Douglas spruce and yellow pine are frequent 

 companions on the southward mountain slupes. the former on 

 the northern slopes in western ^Montana is frequently associated 

 with the western larch. The mixture of the two species obtains 

 only vmder the better watered conditions and diminishing the 

 quantity of larch and increasing that of Douglas spruce even 

 to a pure stand, in the direction of the drier situations. On the 

 other hand the larch sometimes attains pure stands on the cooler 

 and better watered slopes. 



The western larch (L. occidentalis) has several distinctive 

 characteristics. Certain positive qualities stand out lonspieu- 

 ously and certain negative ones are no less prominent. The posi- 

 tive factors in the organization of this species may be summed 



