5, 4, 3, 2, 1. The upper part of the curvature has been worn off, so that the strata appear at the surface a, a, in an

unusual order of superposition.

a folded axis.

100. Strata are deposited horizontally in obedience to gravity and some of them retain the horizontal position, but most strata are inclined to the horizon, having been elevated to various

angles, by subterra

Such an instance is called

Fig. 54.

Horizontal Strata.

nean forces, since their deposition.

four strata deposited horizontally.

In Fig. 55, we have Figure 56 presents the

same strata elevated by subterranean forces, with the upper

most ones rent.

Subsequently at

mospheric and

aqueous agencies

wear off the

upper

Fig. 55.

α

Fig. 56.

portion of the bent strata, which now appear inclined to the

makes with the plane of the horizon; and is reckoned from

Fig. 58.

0° to 90°. When the dip is 90° the strata are, of course, vertical. In the Isle of Wight is a series of strata eleven hundred feet thick in this position. The example in Fig. 58-strata in Wales, on which Powis Castle is built, is given by Mr. Murchison, in his "Silurian System." Vertical strata occur on a much larger scale in the cliffs of Savoy. Strata of calcareous shale in the Alps stand vertical for more than one thousand feet in depth and then curve round to their appropriate position.

Vertical Strata.

101. The dip of the strata is determined accurately by means of an instrument called a clinometer; but it may ordinarily be estimated with sufficient accuracy by the eye. As a general fact, the deepest strata are most highly inclined. The direction in which the edge of an upturned stratum appears at the surface is called the strike or bearing. If a ridge runs North and South, the dip of the beds is East and West, and their strike or bearing North

and South. A pocket compass will enable the observer to determine the strike. The line of dip is always at right angles to that of strike. Horizontal strata have neither dip uor bearing. If we place a book upon the table with the edges of the leaves downward, as in Figure 60, and Fig. 60. remove one cover a short distance from the leaves, this cover may represent a dipping stratum, the dip becoming less and less as the cover is raised, until it becomes horizontal-parallel with the table-when the dip ceases. The back of the book, a a, exemplifies the strike. The anticlinal line or axis is a line along the summit of a ridge or mountain range, from which the strata dip in opposite directions. If both covers of the book be thrown partially open, the anticlinal axis will be represented by a line along the back of the book. The synclinal line or

axis is the line in a valley toward which the strata dip. To represent this, turn the book over, placing the back upon the table, open it partially and the line between the pages will present the synclinal line.

102. The dip is usually easily discerned, but as the edges of highly inclined strata may give rise to horizontal lines on the face of a vertical cliff, as seen by an observer in the line of their strike, their dip would not be apparent. A break in the cliff, giving a section of the strata at right angles to their strike, would at once discover their dip. Thus the strata in the headland, Figure 61, would appear perfectly horizontal to an observer in the boat directly in front, while a person on the shore facing a section at right angles to the strike of the strata, would at once perceive that they dip 40°. The abrupt termination of strata in a Fig. 61.

Apparent horizontality of inclined Strata.

headland is called an escarpment. When the strata dip in all directions from a point, as around a crater of a volcano, the line of strike is circular or elliptical, and the dip is said to be qua-qua-versal. When the strata come out at the surface they are said to outcrop.

92

CONFORMABLE AND UNCONFORMABLE STRATA.

103. An assemblage of rocks formed under the same circumstances, consequently possessing some characteristics in common, is called a formation. It often embraces different substances; the Lias formation includes the Lias limestones, shales and marls, as does the Coal formation the rocks associated with the coal. The time during which such a group was formed is called a geological period. When successive strata or groups of strata are parallel to each other, they are said to be conformable; when not parallel, they are unconformable. In Figure 62, the strata a b c d are conformable, as are also e f g h; but the two groups or formations are unconformable. This indicates Fig. 62.

K that the

Conformable and unconformable Strata.

group e f g h had been formed and elevated before the other group was deposited upon them. As the stratification of different formations is usually unconformable, it is inferred that there have been several different periods during which the various formations were deposited and elevated. The elevation of the strata has not always been perfectly equable; hence fissures occur, on either side of which portions of the same stratum are found at different