CONSTRUCTION OF THE EARTH. 



135 



strata would appear to be horizontal, when in reality they 

 lie at an angle of about 45, and gradually become verti- 

 cal as you go toward a. 



218. Dip and Strike. These are terms which are much 

 used in geology, and therefore require explanation. The 

 line of variation of strata from a horizontal line is called 

 their dip. For example, in the case shown in Fig. 60, if 



the angle made 

 by the lines of 

 the strata, b #, 

 with the hori- 

 Fi - 60 - zontal line, , be 



45 degrees, these strata are said to dip 45 degrees. The 

 strike, on the other hand, is the line of direction in which 

 the edges of the strata run. The meaning of these terms 

 can be made clear by a simple illustra- 

 tion. If you place a book on the ta- 

 ble, as seen in Fig. 61, with the edges 

 of the leaves downward, and move 

 one side of the cover away from the 

 body of the book, a line from the back 

 of the book straight down the cover, 

 5, will be the line of dip, while a line 

 along the back of the book, a , will 

 be the line of strike. 



219. Anticlinal and Synclinal Lines. An anticlinal 

 line is a line along which the strata dip in opposite di- 

 rections. This may be exemplified by placing the book 



in the position seen in Fig. 62. The 

 line along the back of the book repre- 

 sents the anticlinal line. The syncli- 

 nal line is the reverse of this. To rep- 

 resent it, let the book be placed in the 

 position seen in Fig. 63 (p. 136). The 

 line at the angle of the two parts of 

 the book is the synclinal line. These 

 lines do not, however, run along sharp 



Fig. 61. 



Fig. C2. 



