DIP. STRIKE. 



147 



DIP. The inclination of strata from a horizontal position 

 is called their dip, the amount of the dip being the quantity 

 of the angle which the line of inclination makes with that of 

 the horizon, as in the accompanying figure, 78. If the 

 angle made by the meeting of the lines of the strata, 5 5, and 

 the horizontal line, a, be equal to 45 degrees towards the 

 east, the strata are said to dip 45 degrees in that direction. 



FIG. 78. 



STBIKE. Again, the various terms of the dip, the strike, 

 &c., of strata, will be farther understood by the following 

 illustration. The dip, as before observed, is the line which 

 the strata makes with the horizon : the strike is a line at 

 right angles to the dip. In other terms, if the student only 

 place a book on the table, with the edges of the leaves down- 

 wards, and the back of the book up- 

 wards, as in the accompanying figure, 

 and if he move one side of the cover 

 a short distance, the cover so moved, 

 5, will represent the line of dip, while 

 the back of the volume, a, a, will ex- 

 emplify the line of strike. If the 

 cover of the book be extended only in 

 a slight degree, the dip will be propor- 

 tionately steep ; if the cover be opened 

 farther, so that the book lies nearly 

 flat on the table, the dip will be proportionately less. 



Having ascertained the line of dip, we can determine 

 the direction of the strike ; if the dip be towards either the 

 north or south, the strike will be east and west ; if, on the 

 other hand, the dip be east and west, the strike will be 

 north and south. But the converse of this proposition 

 by no means holds good; for though the line of dip gives 

 the line of strike, the line of strike does not give the line of 

 dip, since there are two lines of dip common to every line 

 of strike, and strata having a line of strike running from 



L2 



FIG. 79. 



