23 
Before proceeding to characterise these districts, it may be desirable to 
dwell for a short time on the general features of the vegetation of the 
three climatic zones. 
As before stated, the lower, or with reference to the whole of Britain, 
the midagrarian zone may be considered to comprise all that portion of 
the surface which does not attain an elevation above the sea of about 300 
yards. Consequently, it includes the whole of the central vale : on the 
east, the vales of Cleveland and Pickering and the wide extent of the sur- 
face of the oolitic moorlands, with the exception of the summit of Boseberry 
Topping and a few other detached peaks ; and also a broad branched 
serpentine strip which forms the watershed between the rivers Derwent, 
Swale, Leven, and Esk : and on the west, narrow at first, but widening as 
the rivers descend, the valley of the Tees to the neighbourhood of Hol- 
wick and Mickleton, Swaledale to the vicinity of Muker, and Wensleydale 
to the junction of the two forks of the Ure near Hawes. The great body 
of its vegetation is made up of the species enumerated throughout the 
work as belonging to the British type of distribution : but these mostly 
ascend also into one or both of the upper zones. But those which make 
up the three austral types (i. e. English, Germanic, and Atlantic,) seldom 
pass up above the limits of the present zone. Some of them are com- 
paratively plentiful, and are probably distributed at intervals throughout 
almost the whole extent of its surface. As examples of this class, I may 
mention Cornus sanguinea, Pulicaria dysenterica, Ranunculus arvensis, 
Bryonia dioica, Rhamnus catharticus, Scrophularia aquatica, Acer cam- 
pestre, Convolvulus sepium, Tamus communis. But others, such as most 
of the species previously mentioned as finding their boreal limit in the 
county, are much more restricted in their range. The flora of this zone 
is also characterised by the comparitive rarity within its precincts of 
many of the more or less boreal species, as Trientalis europsea, Primula 
farinosa, Listera cordata, Empetrum nigrum, Habenaria albida, Gnaph- 
alium dioicum. 
The middle, or with reference to the whole of Britain, the superagrarian 
zone includes, on the east, the detached peaks before mentioned, and 
the broad branched serpentine strip forming the watershed between 
the rivers Derwent, Swale, Leven, and Esk: and on the other side 
of the great central vale, that considerable portion of the western 
