8oo PRINCIPLES OF STRATIGRAPHY 



40,cx)o feet of strata, partly of marine and partly of continental 

 origin, were deposited. It is evident that there must have been a 

 gradual downbending of the old land to permit this extensive series 

 of deposits to accumulate. This depression should not, however, be 

 regarded as primarily of tectonic origin. It is much more likely 

 that it represented the slow sinking of the crust under loading and 

 that its formation was due to the progressive reestablishment of 

 isostatic equilibrium. The foredecps, which are situated off the 

 continental margins, are probably of a different character. Here 

 we have in general downward-bending troughs, next to the land 

 mass, where, however, there is no great amount of deposition. Such 

 down-warping of a part of the ocean bed may well be regarded as 



Fig. 184. Anticlinorium. Generalized section in the Alps. (After Heim.) 



of tectonic origin, serving to relieve the accumulating stresses in 

 the earth's crust. An examination of the ocean bottom charts will 

 locate the existing fpredeeps. (See further, Chapter XXIII.) 



Relation of dip, strike and outcrop. 



Dip has already been defined as the inclination of the strata from 

 the horizontal. Strike may be defined as the compass-bearing of the 

 line of intersection which the stratum in question makes with a hori- 

 zontal plane. It may also be spoken of as the compass-bearing of 

 the edge of the inclined bed when cut off by a horizontal plane. 

 The direction of strike is determined by compass, with reference to 

 the true north and south line, i. e., the meridian. A wrong direc- 

 tion is often furnished by the outcrop of the stratum on a 

 sloping surface. If the slope of the surface and the dip of 

 the strata are in the same direction, varying only in amount, or if 

 the slope and dip are in exactly opposite directions, no difference 

 will be observed between outcrop and strike. (Fig. i86a.) Again. 

 if the strata stand vertically, no difference will be observed between 



