152 
PHYSICAL GEOGRAPHY 
both as to dynamical agency and geological period, provided the level 
of land and sea is supposed to be the same now that it was before the 
valleys were made, or else to have varied according to a certain law. 
By these principles we shall be guided in the following inquiry. 
The Penine chain is a summit of drainage, and the course of its 
longest streams is eastward, as exemplified in the Wear, Lune, Greta, 
Swale, and Upper Yore. There is, however, another principal direc- 
tion observable in the Tees, Arkle, Lower Yore, Nid, and Washburn, 
and parts of the Upper Wharfe. We shall see hereafter that the direc- 
tion of the streams, though primarily dependent on the dip of the 
strata, is subject to the controul of other circumstances. The Tees 
the Swale, the Yore, the Nid, the Wharfe, and the Aire, which have 
their sources in the Penine chain, take separate courses through the 
magnesian limestone, but unite into one stream before reaching the 
Humber. Several streams of shorter course also break through that 
limestone, near Catterick, at Crakehall, and near Ripon. 
In general the dales which ramify amongst the mountains of the 
1 emne region commence at the summit of drainage, in hollows which 
are some hundreds of feet below the summits of the hills. It is t 
be observed that we are here speaking not of the actual water channel 
which often are traceable nearly to the very highest ridges, but of the 
valleys which yield them a passage to the sea. This important dis- 
tinction ought never to be neglected in works on Physical Geography 
It frequently happens that the dale-heads of opposite drainages meet 
in a hollow boggy surface, which receives the rills as they gush from 
the mountain slopes, and yields at length two considerable bodies of 
water running in different directions. Thus Tynedale and Teesdale 
Yoredale and Edendale, coalesce at their upper extremities. 
It is also generally observable that the dales as they pass from the 
higher ground grow deeper continually for a certain distance, and 
expose along their sides lower and still lower strata ; afterward this 
slope of the valley diminishes, and the stream passes successively over 
