136 The Plant World. 



may be said that the two mountain sides are geologically of the 

 same origin and chemically of the same general constitution. 

 The entire mountain mass appears to be composed of granitic 

 and gneissic rocks, and mica-schist. Its orographic genesis has 

 given it a steep slope on the east, possibly due to an ancient fault 

 scarp. On the west a more gentle one prevails, with a different 

 lay of the rock, which may influence to some extent the amount 

 and distribution of the soil and ground water. It is probable 

 that the m.ore or less horizontally cleft bed-rock and steep slope 

 of that portion of the east side most particularly examined, give 

 rise to rock talus, water conduits, and deep crevices filled with soil 

 which m.ay, in some cases undoubtedly do, account for the pres- 

 ence of some of the additional species, and other differences. 

 Very little soil water comes to the surface on the west, where the 

 rocks lie more flat and impervious, while a number of cool 

 springs are found scattered over the east side, giving rise to 

 characteristic spring societies. That these edaphic features are 

 by no means accountable for the greater part of the floral con- 

 trast, however, is shown on certain portions of the west slope 

 where escarpments, not unlike some smaller ones on the east, 

 appear without changing the general aspect of the flora. While 

 the east slope is steeper, it has, nevertheless, a deeper and more 

 humous soil, in itself suggestive of better moisture conditions. 

 This is possibly due primarily to certain physiographically de- 

 termined climatic factors named below, and secondarily cer- 

 tainly to the reaction of the plant covering upon it, this in turn 

 reacting upon the plants themselves. 



Aspect, so far as identified with insolation, such a powerful 

 factor on north and south slopes, likewise plays little part here. 

 To be sure the sun rises from one to three hours earlier on the 

 east, while copious dew is present on clear mornings, during 

 which time there is probably very little moisture loss from either 

 plants or soil. It also sets about three hours earlier, at about 

 three to four o'clock p. m., during the growing season. This is 

 not long after the daily heat maximum is reached, which is 

 usually near one o'clock p. m., unless depressed to an extent of 

 commonly 4°-6° F. by temporary noonday clouds, or depressed 

 as well as shifted to the forenoon hours by clouds that last 

 through the afternoon. If light is cumulative near the surface 

 as well as heat, as is intimated in connection with Warming's 



