118 GEOGNOSY AND GEOLOGY. 
in which case they are generally circular or elliptical in shape, and are 
often converted into lakes, as is the case with Derwentwater, or the Lake 
of Keswick (pl. 51, fig. 2), in the county of Cumberland, England. 
Valleys which are half inclosed, generally extend far in a longitudinal 
direction, and have but one outlet, while the open have this on two sides. 
The latter, the open, are also called valleys of interruption, as they generally 
connect two longitudinal valleys, and therefore break through, as it were, 
the separating ridge. , } 
The study of the relations in which the valleys of a mountain system 
stand to each other, is of extraordinary interest, this being increased in 
many cases in proportion as their character enables us to recognise a certain 
causal connexion. This, however, can only be elucidated after a close 
comparison of the relations of stratification has been instituted; we shall 
therefore first consider the relations in which valleys stand to mountains 
and to mountain ranges. Those valleys which lie within the limits of a 
system of mountains are called mountain valleys, in distinction from outer 
valleys lying to the outside of the same systems; both, again, differ from 
intermediate valleys which separate two contiguous systems. Since valleys 
are bounded by mountain ranges or spurs, and as we distinguish three 
kinds of these, it naturally follows that the valleys will also differ among 
themselves. Accordingly we separate primary or longitudinal valleys from 
lateral or cross valleys, as well as from secondary valleys, these being all 
bounded by the corresponding mountain ranges. Longitudinal valleys have 
generally a considerable extension in length, and but little in breadth, the 
surface presenting much uniformity of appearance. The case is precisely 
the reverse with the cross and secondary valleys ; these, on the whole, are 
shorter, and alternately expand and contract, often run out into ravines, 
their walls being not unfrequently formed by remarkable rocks. We also 
find here the peculiar terrace-like character, with the accompanying 
waterfalls. In mountain chains the corresponding valleys exhibit a more 
or less parallel arrangement; in mountain groups they exhibit a radiation 
more or less complete. A phenomenon of no very rare occurrence is 
the mutual intersection of valleys at different angles. 
4. Plains. 
When valleys are very broad they pass into plains, no well defined limit 
between the two being possible. The character of plains may experience 
modifications by taking their boundaries into consideration. Under this 
point of view we distinguish coast and interior plains. The former are such 
as are bounded on one or more sides by the sea, the latter being inclosed on 
all sides by mountains or mountainous land. These, however, are rather 
geographical distinctions ; to the geologist the division into depressed plains, 
plains proper, and elevated plains (plateaux), is of much more importance. 
The first lie below the level of the sea, the second are elevated slightly 
above it (as the coast of Holland), and the third, sometimes called table- 
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