152 J. E. CRIBBS 



arrangement of the underlying rocks is such as to give rise to 

 numerous springs and a strong seepage of water from many of 

 the slopes. The elevation of this area ranges from about 750 

 feet at Pittsburgh to 1850 feet in southern Warren County. 

 Because of the complete dissection of this region into hills and 

 valleys, it has a topography very unlike the glacial worked 

 soils, and the disposition of the flora is likewise striking and 

 characteristic. Three vertically disposed stages occur here, 

 namely (a) An evergreen forest stage, (b) A mixed (or transi- 

 tion) forest, (c) A deciduous forest. 



The Evergreen Forest 



The evergreen forest occupies the lowest position on the 

 slopes; and extends from their bases to a height of some 150 

 feet, or in some cases even to the height of 350 feet. The soil 

 is decidedly rocky ; and with an abundance of cool water supplied 

 by seepage, produces an environment which is closely similar to 

 that existing in the rocky ravine of the glacial zone. The white 

 pine-hemlock forests appearing here are the most valuable of 

 any in Pennsylvania. The large stands of primaeval pines in 

 Forest and Warren Counties are perhaps the finest large tracts 

 of this timber available; and from an ecological as well as econo- 

 mic standpoint are of the greatest interest. They are the 

 deepest shaded forests in western Pennsylvania and necessarily 

 demand of the undergrowth a high abihty for development under 

 poor light conditions. 



These forests are striking in that the number of species rep- 

 resented is very low, and, because on the whole, the floral 

 composition is less variable than that of any other situation. 

 The lower portion of the evergreen zone is dominated by Tsuga, 

 which develops forming a very close stand, or even one which is 

 quite pure. Betula lenta, and Betula lutea, are the only decidu- 

 ous trees represented here, and occur along the borders of the 

 streams and in the small ravines upon the slopes. The lower 

 strata of the forest like the upper, are prevailingly evergreen; 

 the dominant species being but four in number, namely, Lyco- 



