STATE (iKOLOGIST. 48 



ttans from tho renter, [f it is a ridge, they dip to the right and 

 left of the axis. This ridge may have been planed down to tho 

 il level of the country. It' this is the case, we shall then, 

 . from the central lino either to the righl or left, 

 continually from lower to higher rocka, withonr changing onr 

 tion. We ascend Btratigraphically, but not topographi- 

 cally. 



This ridge may not pursue a straight course It may finally 

 bend round, and proceed in a direction parallel with itself. It 

 is obvious then, that the strata between the two portions or 

 branches of the ridge, form trough-shaped depressions. In 

 many cases all the edges of the over-lying- strata are turned 

 up. and they rest in a dish shaped depression. When the 

 irregularity of the original elevations is considered, it is obvi- 

 ous that the eutcropping edge of any stratum, when traced 

 over the surface of the earth may pursue a very tortuous 

 . or strike. It is also obvious that the width of the stra- 

 tum at tho surface will be more, if the surface cuts it very 

 obliquely, less, if the surface cuts it nearly at right angles. 

 This depends, in other words, upon the amount of the dip; so 

 that a thick formation, by being nearly vertical, may occupy a 

 very narrow belt of country; while a thin one, by being nearly 

 horizontal, may occupy a belt several miles in width. 



All this is familiarly illustrated by the lines of the " grain " 



of l smoothly planed board, especially if slightly gnarly or 



knotty. The knots may represent the granite, while the layers 



of wood surrounding it — here apparently thin, because cut 



nearly at right angles, there spreading out, bocause cut more 



obliquely, here running in a straight line, and there tracing a 



ig path — may represent the layers of rock, occupying a 



■A position above the granite. 



These explanatory observations are here admitted, in the 



f obviating some difficulties almost always experienced 



- onversed in geology, in forming general concep- 



pcal Structure of a particular region. ( 



The wide interval between the Alleghany and the B 



