COURSES OF THE EARTH s MASONRY. 77 



dip away from them. Next, a few miles farther from Cin- 

 cinnati, on all sides, we come to the Devonian strata; and 

 next, the Lower Carboniferous. 



Very frequently the dip is in opposite directions along a 

 line, as if an inverted nest of troughs had had their bottoms 

 sawed off. If you turn an open book so that it rests on the 

 table with the back up, then the leaves are strata ; their in- 

 clination to the table is the dip, and the two inclinations in 

 opposite directions form an anticlinal structure. If you keep 

 the leaves in the same position and turn the back of the book 

 down, the structure is synclinal. It is rather necessary to un- 

 derstand these terms, because we meet with such structures so 

 frequently, and shall have to talk about them. 



Very commonly, the dishes and troughs of which I am 

 speaking are irregular. A trough, whether inverted or not, 

 may bend, and change the course of its axis. That makes it 

 more difficult to follow, especially as nearly all the rock sur- 

 faces are concealed by Drift. Sometimes the trough is de- 

 pressed at one end ; sometimes at both ends ; sometimes in the 

 middle. Again, there may be an uplifting of one or both 

 ends, or of the middle. The determination of the order of 

 the strata is often much complicated by those erosions of which 

 we have talked. Suppose, for instance, we have an anticlinal 

 axis, and suppose the surface of the earth horizontal. Then 

 if a deep broad valley were worn across and through the an- 

 ticlinal series of strata, what sort of curves would be pre- 

 sented by the cut edges of the formations? Can you think 

 them out? But suppose the anticlinal strata are elevated in 

 a long ridge like a mountain, and a deep valley should be 

 cut down one side, can you picture to your imagination the 

 lines which the cut edges of the strata would trace ? I think 

 it would be well for the ingenious reader to contrive something 

 to serve as a model to illustrate these complicated arrange- 

 ments. A nest of wooden or paper dishes might be glued 

 together and sawed and grooved and carved into shapes 

 imitating the configuration of the earth's surface. Even 

 in level and undisturbed strata erosions have created some 



