364 GEOLOGY. 



porary. Thus deltas (p. 198), though not wholly the work of sea- (or 

 lake-) water, often constitute irregularities of a more or less permanent 

 nature. This is the case where they project beyond the general trend 

 of the coast-line. Where, on the other hand, they are built at the 

 heads of bays, they tend to simplify the coast-line by obliterating the 

 indentation. The delta at the head of the Gulf of California is an 

 example. So too is the delta of the Mississippi, the real head of which 

 is far above the present debouchure of the stream. The form of the 

 delta in ground-plan depends on the horizontal configuration of the 

 coast where it is developed, on the strength of the waves and shore-cur- 

 rents, and on their relation to the amount of detritus contributed by 

 the stream concerned. Good illustrations are furnished by the Gulf 

 of Mexico where the deltas of the Mississippi and Rio Grande are in 

 contrast. 



So far as concerns the vertical configuration of coasts, erosion and 

 deposition are in contrast, for while the former tends to develop steep, 

 irregular, and often high slopes (p. 349) from the land to the sea, the 

 latter tends to develop gentle, regular, and low ones. A partial excep- 

 tion to the latter part of this general statement comes about through 

 the building of dunes, the material for which is furnished by the waves. 



SUMMARY OF COASTAL IRREGULARITIES. 



The horizontal irregularities of coasts are both large and small. 

 Some of them, like Florida, Sandy Hook, etc., consist primarily of pro- 

 jections of land into the sea; others, like Chesapeake Bay, the Gulf 

 of Mexico, and Puget Sound, are projections of the sea into the land; 

 while still others, like the Gulf of California and its associated peninsula, 

 cannot readily be put in either of the foregoing classes. Some of the 

 irregularities of the land border, such as Yucatan, are more or less 

 nearly normal to the general trend of the coast which they affect, while 

 others, such as the '^ beaches" along the Atlantic and Gulf coasts of the 

 United States (Figs. 319 and 320), are more or less nearly parallel with it. 

 Some of the irregularities, especially some of the small ones, are more 

 or less angular in their outhne (PL XX and parts of Fig. 2, PL XXII), 

 while others are bounded by curves instead. 



In many cases more than one factor has been involved in the develop- 

 ment of irregularities. In the case of great irregularities, diastrophism 



