H2 or iii! i.o-GEOLo<;y 



field suffices to show how constantly the dip of strata 

 varies, now to one side, now to another, every Midi 



ion producing a corresponding change upon the 

 line of strike. Where the deviations arc slight, and of 

 local character, while the mean direction of inclination 

 remains the same, we take that mean direction as 

 governing the strike (as at hand !" in l-'ig. 2,S). \\ 

 on the other hand, the dip is to different points of the 

 compass in succession over wide spaces, we connect the 

 arrows on our map by lines (as in band ( ; in I i^. 28 1, and 

 find that the strike becomes a curxed, and even, it may 



very sinuous one. 



Difference between Outcrop and Strike.- Unless these 

 two terms, already explained, are clearly distinguished in 

 constructing a geological map, we shall either lose t he- 

 impression of the external form of the ground, which a 

 correctly-traced outcrop so often vividly conveys, or we 

 shall be in danger of regarding the dip as constantly 

 changing, and the strata, though perhaps nearly flat, as 

 extensively disturbed. Looking at any good geological 

 map of England and Wales, the reader will notice that 

 the bands of the Oolitic and Cretaceous rocks, while 

 retaining a tolerably persistent strike from south-west 

 to north-east, across the breadth of the country, pre- 

 sent most sinuous and irregular edges. The direction 

 of the dip, and consequently the trend of the strike, 

 change but little, yet it xvill be observed that the 

 outcrop is continually shifting to and fro. The strata 

 really follow each other in parallel bands. If we 

 could plane down the whole country to a dead 

 level, these bands would be marked by alternate ap- 



