STUDIES FOR STUDENTS 



being that the course of the stream has been conditioned by the 

 gaping of the joint fissure, and he finds in some instances that 

 the beds on opposite sides of the stream stand at the same level. 

 Lowl takes account of the lateral translation of streams working 

 in inclined strata, and argues that the bed of the stream does 

 not represent the position of the fault, which in many cases can 

 be observed high up upon the slope. Speaking of the valley of the 

 Oder in the Harz mountains, a stream which follows the direc- 

 tion of the Oder fault, he says : 



The Oder valley is therefore nothing more than a performance of the 

 flowing water. Its course along a line of disturbance proves only that this 

 directed the erosion into a definite course. 



The writer, by taking account of the great number of parallel 

 faults within a series, would ascribe the courses of the streams 

 within a network more largely to the presence of many long and 

 narrow down-thrown orographic blocks (Graden), which have 

 conducted the water like canals. 1 Upon this assumption the 

 equal altitude of the strata upon opposite sides of a valley does 

 not preclude the probability of faults along the bed of the chan- 

 nel. Whatever explanation be offered of the directing of 

 streams upon a large scale by joints and faults, there can be no 

 doubt that streams are of greatest service to the geologist in 

 showing the directio?is rather than the positions of lines of dislo- 

 cation. Regular networks of streams may be assumed to be 

 much more common than the literature of the subject would 

 indicate, for the reason that comparatively little attempt has 

 been made to explain stream directions on the basis of structure 

 planes of the underlying rocks. 



5. Zigzag course of coast lines. — Evidence of the same kind 

 as that derived from drainage systems is sometimes to be sup- 

 plied by coast lines, which may disclose the lineaments of a 

 submerged drainage system, the border of a sunken composite 

 orographic block, or a margin of areas of outcroppings. The 

 coast of Scotland between Sutherland and Ross furnishes an 

 illustration. 2 



1 Twenty-first Ann. Report U. S. Geol. Survey, Part III, p. 149, PL XVI., Jour. 

 Geol., Vol. IX, p. 483. 



2 Judd, "The Secondary Strata of Scotland," Quart. Jour. Geol. Soc. (1873), pp. 

 131, 134, PI. VII ; see also Suess, op. cit., Vol. I, p. 269. 



