BURTON: THE STORY OF THE LINCOLN GAP, 275 
Rivers may be roughly divided into two main classes, ‘primary 
and ‘secondary.’ The ‘primary’—or, as they are sometimes called, 
‘transverse’ rivers from their running with the dip, transverse to the 
strike of the beds—take their rise on any elevated ground, and, 
having a gradual slope towards the sea, cut in a more or less direct 
line, through hard ridges and soft strata alike ; and the valleys they 
form in their note ok with the aid of rain and atmospheric agencies, 
are known as ‘transverse’ valleys ; while the streams that flow into 
them on the sides, and which follow the strike of the strata, cutting 
through the softer and weaker beds between the ridges, are known as 
‘secondary’ or ‘subsequent’ rivers, and their valleys longitudinal. 
These ‘subsequent’ streams may be, and they sometimes are, of 
greater length than the ‘ primary’ rivers; and, as they deepen their 
beds and widen their valleys, they leave the hard ridges, parallel to 
which they run, standing out as inland cliffs or escarpments: (the 
formation of ‘longitudinal’ valleys, and of inland escarpments, being, 
in fact, in each case the result of one and the same process). Again, 
as time goes on, and the ‘longitudinal’ valleys are pushed further 
back, these subsequent rivers sometimes succeed in tapping, or 
capturing, other rivers and streams flowing at a higher level than 
themselves, which they happen to reach. 
The Trent was a ‘primary’ stream when it first started on its 
course from the high district in the west—a time when the Derby- 
shire hills were hundreds of feet higher than they now are—and, 
finding a gradual slope towards the east, thither, of necessity, it 
directed its steps, cutting through eaaping ridges and the more 
yielding strata alike till it reached the se 
The Humber, too, was a ‘ primary’ river when, ages ago, it left 
its cradle in the Workehite hills; and in its lower course it is one 
still, or rather the beheaded remains of one, for its upper streams— 
which Prof. Davis thinks may have been somewhere about Halifax or 
Huddersfield—are lost. This river, as it reached our land, had the 
Same ridges to cut through as the Trent—the Triassic, Oolite, and 
Chalk—and it, too, found an outlet in the eastern sea. 
At that time, however, as now, it lay at a lower level than the 
Trent ; and, as it deepened its bed, a ‘longitudinal’ valley began to 
form on the soft Keuper marls to the south, where the Trent now 
flows. 
The river Idle, which runs into the Trent a little to the north of 
Gainsborough, and was then an independent stream draining the 
land round Mansfield where it rises, flowed down this valley, deep- 
ening and widening it continually, till it reached the Humber. Other 
brooks and rivulets, collecting from the land around, helped on the 
Sept. 18 1895. 
