48 VEGETATION OF THE PEAK DISTRICT [cH. 
rather remarkable therefore that the plant is not indigenous in 
the Pennine woods at the present time. Still, judging from 
the paucity of the number of the records of buried pine 
timber, the species does not appear to have been more than 
an occasional or, at most, a locally abundant integer even 
in the prehistoric woods of the Pennines; and it probably 
became extinct at a very remote date. At the present time, 
seedling pines are not abundant on the Pennines even in and 
around pine plantations; and the tree does not flourish nearly 
so well on these hills as on the dry and sandy heaths in the 
south of England. 
The only species of poplar which is indigenous in this 
district is the aspen (Populus tremula); and even this species 
appears to be quite rare in the oak and birch woods. Other 
poplars (e.g., *P. canadensis and *P. candicans) are planted 
occasionally, though more frequently on the outskirts of the 
woods than inside the woods themselves. 
Several indigenous species of willow occur. Salix cinerea 
is common, and ascends to nearly 1200 feet (366 m.) in some of 
the cloughs. S. caprea is occasional; but both S. cinerea and 
S. caprea are absent from the driest woods. S. aurita is local, 
but occasionally forms thickets in damp spots in the cloughs. 
S. pentandra is also rare. Hybrids of S. caprea, S. cinerea, 
and S. aurita are not uncommon. S. repens occurs, but js 
rare. S. fragilis, S. viminalis, S. caprea x viminalis occur 
locally by the stream sides, at altitudes below 600 feet 
(188 m.). Although 8S. alba, S. purpurea, and x S. rubra 
(=S. purpurea x viminalis) are recorded (Linton, 1903), they 
are perhaps not indigenous in the Peak District. 
The hazel (Corylus Avellana) is rather abundant in the 
damper woods, but much rarer in the drier ones. 
The common birch (Betula pubescens) is, on the whole, the 
most constant and the most abundant associate of the sessile 
oak. Forms or varieties with glabrous or sub-glabrous twigs 
(B. pubescens forma denudata) are not uncommon: B. pubescens 
var. parvifolia is rare, but has been observed. In some of the 
woods, however, the birch is rare or absent. Not infrequently, 
the birch becomes locally dominant in places where extensive 
felling of the oak and no subsequent planting, have taken 
place; and its small, light, and winged fruits are evidently of 
