412 An Essay on Rents. [Duc. 
elementary substances are not the same, nor their proportions to one 
another equal, in every part, the parts which differed from others 
diminished in bulk more or less than they. Hence some parts have 
sunk much lower than others; but as in general that difference in 
the component parts of matter reached its maximum progressively, 
it is productive of the undulated surface before described, under 
which the matter at a,c, e, sunk lower than at J, d, f. 
The shape of mountain and valley just mentioned appears to be 
the most general, though not the only, one: for when the matter 
under valleys sunk below the tops of mountains above a certain 
distance, it appears to have separated the mountains into two or 
more parts. Let fig. 5 be the cross section of a mountain that is 
broken into two parts, A, C. Let a lc be the general surface line 
of a mountain when it preserves the ordinary figure. But the. 
matter under the vallies a, c, sunk lower than this line by the depths 
ck and ad; and in doing so it forced the mountain A C to separate 
in the middle at J g ; and one side to pass from l g to e g, and the 
other from lg to ig. The valley B, therefore, is formed by the 
separation of the mountain; and the sides e g and gi are not 
natural, but forced, surfaces. As all valleys similar to the valley B 
are filled up to certain heights, say equal to the height of the line 
fh, doubts may be entertained if the sides 7 9 and eg, as seen on 
the surface, really continue to, and meet at, the point g; but suffi- 
cient proofs are found in the appearance of the sand and gravel, 
and in the water of lakes, by which such parts as the part f g h are 
filled. In the middle of the last described valley we sometimes 
meet with a rock, D, fig. 6, rising, as it were, out of the body of 
sand, Such a rock continues downwards till it meets the sides L k 
and h m, at mand k, and the hollows kc d and f gm are filled 
with gravel and sand, or with the water of lakes. ‘he pass between 
Ambleside and Cockermouth, and probably that between the east 
and west seas, which contains the lakes Linnhe, Locky, and Ness, 
consist of a succession of valleys like that valley just described. 
Sometimes a mountain is separated into three parts, as fig. 7 repre- 
sents, In all these separations the opposite sides of valleys are 
seldom of the same height; and sometimes one side of a valley is 
so much lower than the other, as to be totally covered with alluvial 
matter, or with lakes, The valley of Windermere has this appear- 
ance: gf i, fig. 8, is the lake, k / the steep side of the mountain 
on the west,* and g f the flat mountain on the east side of the lake. 
The part ef h was once as high as the part / m; at that time they 
would appear like the pricked sectional lines a, b, c,d; but the 
former part having a tendency to sink lower than the latter, a sepa- 
ration took place between them, and left them at liberty to contract 
independent of each other ; accordingly the part a ) sunk to e fh, 
and the part ¢ d to 7 m; but the part g 47 of the side f h is 
+ The steep rented surface of this mountain is between the Bellgrange and the 
Ferry. Above and below these places the mountain has its natural surface, 
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