GEOLOGICAL SECTIONS. 39 



Strike. Another line, crossing at right angles this 

 line of dip, that is of greatest steepness, must neces- 

 sarily be horizontal it would therefore coincide with 

 the edge of a bed that might come to the surface in a 

 perfectly level tract of country. This seldom occurs, 

 and owing to the general unevenness of the surface an 

 outcrop can rarely be represented by a straight line, 

 although the dip may for a long distance continue the 

 same. But a line through all points, situated at the 

 same level on a boundary, will always be found to lie at 

 right angles to the " dip ;" this line is called the " line 

 of strike," or shortly " strike," of a bed or a formation. 



From a consideration of the above, it will be seen 

 that as the " direction of dip " varies, so also will the 

 " line of strike," which is roughly the outcrop of a for- 

 mation : inversely, as the " line of strike " or outcrop 

 varies, so also will the "direction of dip" in equal pro- 

 portion. Of this we may at all times be certain, even 

 although no ocular evidence be forthcoming, and the 

 fact will frequently be found to aid us in our geological 

 investigations. 



It is a self-evident proposition, that as the amount of 

 " dip " varies, so does the horizontal breadth that the 

 outcrop of a bed of constant thickness, will occupy at 

 the surface. But it is a very useful thing to remember 

 in geological surveying, also in drawing sections where 

 the evidence is scanty, especially as for the latter pur- 

 pose the actual slope of the ground is ascertained. It 

 follows, that the value of any two of the three factors 

 tabulated below being known, that of the third can 

 easily be ascertained. 



1. The thickness of a formation. 



