Fig. 60 Dip and Strike.—In describing the manner in which strata depart from their original horizontality, some technical terms, such as “dip” and “strike,” “anticlinal” and “synclinal” line or axis, are used by geologists. I shall now proceed to explain some of these to the student. 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. 60), 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. 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. Fig. 61: Apparent horizontality of inclined strata. If the upper surface of a hard stony stratum be uncovered, whether artificially in a quarry, or by waves at the foot of a cliff, it is easy to determine towards what point of the compass the slope is steepest, or in what direction water would flow if poured upon it. This is the true dip. But the edges of highly inclined strata may give rise to perfectly horizontal lines in the face of a vertical cliff, if the observer see the strata in the line of the strike, the dip being inward from the face of the cliff. If, however, we come to a break in the cliff, which exhibits a section exactly at right angles to the line of the strike, we are then able to ascertain the true dip. In the drawing (Fig. 61), we may suppose a headland, one side of which faces to the north, where the beds would appear perfectly horizontal to a person in the boat; while in the other side facing the west, the true dip would be seen by the person on shore to be at an angle of 40°. If, therefore, our observations are confined to a vertical precipice facing in one direction, we must endeavour to find a ledge or portion of the plane of one of the beds projecting beyond the others, in order to ascertain the true dip.