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.
Fig. 68.
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.