520

FIELD SKETCHING.

Plate 111.

Plate I.

Plate III.

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itself,—the valley and the ravine, the river and the streamlet, the lake and the pool,and nearly all the topographical features of aqueous formations acted on by water,both as to original formation and to subsequent abrasion. The defects lie in theserepresentations being somewhat exaggerated where the mud is very soft and the bankrather steep ; and also in the absence of such main features as are given by the boldintrusions of igneous rocks; though, when these are modified, as in D, by aqueousagency, they are represented with considerable fidelity.

"With regard to Class D, remarkably defined and instructive studies from naturewill be found in the Irish mountains, which are nearly all disposed in isolated groups,separated from each other by flat lands, a large proportion of which is bog.

On whatever scale the subjects of study may be,—

The master-lines of ground are,

1st. The main or summit ridges of the mountain"! _ . . , . „

or hill . . . . ! Referring chiefly to

_ , ..* | classes B and D.

2nd. a he water-courses . . . . . J

3rd. The coast or horizontal contour lines . . Referring to all.

The subordinate lines are those more or less oblique contour lines defining theminor features, and generally called feature lines.

ORDERS I. AND II.

THE RIDGE AND THE ’WATER-COURSE.

In aqueous formations there is always a close conformity of parallelism betweenthese two lines, as in Plate I. fig. 4, between no and ll' or mm'. In those ofigneous character there may be only a very general one between the crest of theupper regions and the lines at the base; though, as we descend, this parallelismincreases till the lower grounds are reached, which being usually composed ofB or D, again admit of the above-mentioned “ close conformity.”

In reference to these two orders of lines, the first thing to be done in rock andclay formations (excluding both diluvial* and alluvial flats from the latter) is to studywell the general character of the district in relation to the nearest mountains orextensive hill range, so that the representations may have the force of truth, and bew r ell characterized by a significant reference to the whole, of which it is hut apart, as far as the extent of ground permits. This is indispensable in plans ofconsiderable tracts of mountainous or hilly country; and where several persons areto be employed in its execution, there must be one guiding head, to give this speakingcharacter, and one qualified to insure its observation in all the performances of theassistants. In small portions of ground, such as a camp, an ordinary position, orbattle-field, &c., these considerations are not quite so important; but still the spirit,clearness, and simplicity of the work, will always be more or less dependent on thisprinciple being borne in mind.

In formations of loose sand and gravel (like those extending from the east ofHolland , for upwards of 1000 miles, across the north of Germany , into the heart ofRed Russia, skirting the northern slopes of the Hartz and Bohemian mountains,without a hill exceeding 400 feet in height,) this relationship between the mountainrange and its lower members soon disappears; and the character of such ground isbest obtained by studying carefully delineated charts of shallow seas, or like forma-tions, such as those of the German Ocean, especially towards the coasts of Holland and the Straits of Dover ; the hills being in this case without any very obviousrelations, either as successive and subordinate heights, or as to lateral connection.

* This expression is still retained for poi»t-tertiary level grounds, senior to what is obviously alluvial.