9i 
of the original woods of the habitat. Among the other trees to be mentioned are 
Hard Maple, Red Maple, Tulip Tree, Walnut, Hickory, Swamp White Oak, 
Red Oak, and, in fact, quite a large list of species, with also occasional old Syca- 
mores, which may be considered as relicts of the preceding association. The 
indications are that the moss flora was about the same as in the woods of the 
ravine slope ( see “5 ’) and it was probably even richer. 
There would be good reason to believe that this Silver Maple- White Elm 
forest would eventually give way to a Hard Maple-Beech forest, but the writer 
has seen no examples of such woods, even though they may have been present a 
century ago. 
9. Sambucus racemosa— Hydrangea arborescens Association. Red-Berried Elder- 
Wild Hydrangea Thicket. 
This thicket occurs on the precipitous talus slopes along the larger streams 
where extensive under-cutting has taken place. There are a number of good 
examples of such talus slopes along the Allegheny River above Pittsburgh. The 
soil is very unstable, landslides of various sizes being of frequent occurrence. 
The steeper slope's are usually mainly covered by a thicket with a small scatter- 
ing of trees, but after the cessation of undercutting the slope may become more 
stable and, eventually, the thicket will give way to a diversified forest probably 
best characterized by Basswood and Red Elm, and this steep slope would in time 
become more gentle and pass into a forest of about the same character as that 
of the ravine slopes, the Birch-Maple-Beech-Oak forest. 
On the face of the steep talus slope there are numerous small ledges and 
rock-crevices to which the moss flora is mainly confined. The list is usually 
about as follows: 
Bryum affine (Brid.) Lindb. 
Bryum intermedium (Ludw.) Brid. 
Bryum pallescens [Schleich] Hedw. 
Ceratodon purpurascens (Hedw.) Jenn. 
Dicranella varia (Hedw.) Schimp. 
Funaria hygrometrica [L.] Hedw. 
Leptobryum pyriforme [L.] Wils. 
Webera nutans [Schreb.] Hedw. ( Pohlia nutans Lindb.) 
10. Philonotis Association. 
On the wet rock bottom of the more open ravine below the gorge, where 
often submerged in rainy periods, Philonotis Muhlenbergii and P. fontana so 
often form the most conspicuous vegetation that it has been thought best to name 
the association after them. The vegetation in this habitat is, in fact, mainly 
mosses, there being also Marchantia and Conocephalus, with a very few seed 
plants in the crevices. Selaginella apus is found here quite frequently. The 
moss list is as follows: 
Ambly stegium orthocladon (Beauv.) Jaeg. 
Calliergon cor difolium [Hedw.] Kindb. 
