Between the layers of shale, accompanying coal, we sometimes see the leaves of fossil ferns spread out as regularly as dried plants between sheets of paper in the herbarium of a botanist. These fern-leaves, or fronds, must have rested horizontally on soft mud, when first deposited. If, therefore, they and the layers of shale are now inclined, or standing on end, it is obviously the effect of subsequent derangement. The proof becomes, if possible, still more striking when these strata, including vegetable remains, are curved again and again, and even folded into the form of the letter Z, so that the same continuous layer of coal is cut through several times in the same perpendicular shaft. Thus, in the coal-field near Mons, in Belgium, these zigzag bendings are repeated four or five times, in the manner represented in fig. 67., the black lines representing seams of coal.[53-A] [Illustration: Fig. 67. Zigzag flexures of coal near Mons.] _Dip and Strike._--In the above remarks, several technical terms have been used, such as _dip_, the _unconformable position_ of strata, and the _anticlinal_ and _synclinal_ lines, which, as well as the _strike_ of the beds, I shall now explain. If a stratum or bed of rock, instead of being quite level, be inclined to one side, it is said to _dip_; the point of the compass to which it is inclined is called the _point of dip_, and the degree of deviation from a level or horizontal line is called _the amount of dip_, or _the angle of dip_. Thus, in the annexed diagram (fig. 68.), a series of strata are inclined, and they dip to the north at an angle of forty-five degrees. The _strike_, or _line of bearing_, is the prolongation or extension of the strata in a direction _at right angles_ to the dip; and hence it is sometimes called the _direction_ of the strata. Thus, in the above instance of strata dipping to the north, their strike must necessarily be east and west. We have borrowed the word from the German geologists, _streichen_ signifying to extend, to have a certain direction. Dip and strike may be aptly illustrated by a row of houses running east and west, the long ridge of the roof representing the strike of the stratum of slates, which dip on one side to the north, and on the other to the south. [Illustration: Fig. 68. Diagram.] A stratum which is horizontal, or quite level in all directions, has neither dip nor strike. It is always important for the geologist, who is endeavouring to comprehend the structure of a country, to learn how the beds dip in every part of the district; but it requires some practice to avoid being occasionally deceived, both as to the point of dip and the amount of it. [Illustration: Fig. 69. Apparent horizontality of inclined strata.]