STATE GEOLOGIST. 43 



tions from the center. If it is a ridge, they dip to the right and 

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

 general level of the country. If this is the case, we shall then, 

 in passing from the central line either to the right or left, pass 

 continually from lower to higher rocks, withour changing our 

 elevation. We ascend stratigraphically, 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 sutcropping edge of any stratum, when traced 

 along over the surface of the earth may pursue a very tortuous- 

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

 tum at the 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 a smoothly planed board, especially if slightly gnarly er 

 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- 

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

 geological position above the granite. 



These explanatory observations are here admitted, in the 

 hope of obviating some difficulties almost always experienced 

 by persons unversed in geology, in forming general concep- 

 tions of the geological structure of a particular region. 



The wide interval between the Alleghany and the Kocky 



