1905] WHITFPORD—FORESTS OF FLATHEAD VALLEY 1 iB 
water, and during all the Palaeozoic and part of the Mesozoic the 
pre-Cambrian rocks were subjected to subaerial erosion. A move- 
ment at the close of the Jurassic again brought the sea over it. Then 
Cretaceous sediments of considerable thickness were deposited upon 
pre-Cambrian limestone and quartzite. In post-Cretaceous times 
a fold was overturned to the northeast. Some time in the Tertiary, 
probably during the Miocene, the faulting mentioned above occurred. 
The rocks tilt as a rule to the southeast, the northwest fault face of 
the rocks being very steep. 
SUMMARY. 
1. The Flathead valley is due to a fault, and is technically known 
as a rift valley. 
2. The greater part of it was formerly occupied by a lake. 
3- The present drainage system of the valley consists of the Flat- 
head Lake, the Flathead River and its branches, and the Pend 
d’Oreille River. 
4. The Swan River valley, an arm of the Flathead valley, has 
for its drainage lines the Swan Lake, the Swan River, and the Big 
Fork River with its branches. 
5. These drainage systems are remnants of the former more 
extensive lake that occupied the valleys. 
I. CimMate oF FLATHEAD VALLEY IN RELATION TO CLIMATIC 
FORMATIONS. 
It has already been shown that the climate determines the general 
plant formation of a region. It is important, then, that the elements 
of the climate be analyzed thoroughly, and it should be pointed out 
at the same time in just what way these elements affect the distri- 
bution of plants. In order to obtain a better understanding of the 
true relation of the forests of this valley, its climate will be com- 
pared with that of the northern peninsula of Michigan, where 
both deciduous and coniferous forests are found; and with that of 
the Puget Sound region, where the coniferous type of forest reaches 
its highest development. 
There are meteorological reports from two stations in the Flat- 
head valley. These reports, though meager in some particulars, 
will give a fair idea of the climatic conditions found in a prairie 
