ioc HARSHBERGER: PHYTO-GEOGRAPHIC SKETCH 
and syenitic rocks excludes landslide action and lateral cutting is 
relatively slow, as compared with ravines formed in clay. Thus. 
the conditions are much more favorable for the growth of plants 
in a rock-gorge than ina clay ravine. Rock-gorges are shady 
and often the rocks drip with water and are, therefore, carpeted 
with mosses, ferns and liverworts. Shade-loving plants abound, 
whose leaves are broad and thin. The stages of development 
pass more slowly in rock-gorges. With the gradual widening of 
the cafion, however, the character of the flora undergoes a slow 
change, so that the vegetal covering is never stable, but con- 
stantly shifting, now of one appearance and with the lapse of time 
and change of physiographic conditions of another. 
The character of the soil conditions, therefore, influences the 
particular kind of vegetation, so that we may have with the same 
exposure of light, heat and moisture a different flora, if the super- 
ficial soil deposits are different. Recent work * appears to show 
that, contrary to opinions that have long been held, there is no 
obvious relation between the chemical composition of the soil as 
determined by methods of analysis used and the yield of crops, 
but that the chief factor determining yield is the physical condition 
of the soil under suitable climatic conditions. The rainfall deter- 
mines the productiveness of a country. Temperature and rainfall 
together are one of the most important natural resources of a 
country. 
Clearly, therefore, the distribution of species does not depend 
so much upon the chemical character of two different strata,t but 
it is because one geologic area has advanced further in its life his- 
tory than the other. The vegetation, for example, of a clay hill 
to-day will be seen on a sand hill in the future. The laws that 
control changes in the plant covering of a country are, therefore, 
plainly physiographic. Wherever hills are being eroded, valleys 
widened, rivers deepened, waterfalls eliminated, lakes filled, or 
coastal plains enlarged, there must be a constant change in the 
plant societies, or a succession in definite order of plant groups. 
* Whitney, M. and Cameron, F. K. The esting vt the soil as related to crop 
production. U.S. Dept. Agric. Bureau Soils, Bull. 903. 
+ Cowles, H. C. The influence of the cern paw on the character of the 
vegetation. Bull. Am. Bureau Geog., Je- 
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