CHARACTERISTICS 609 



of regularity in spacing is a most characteristic feature of well developed 

 forms and must be carefully considered in any attempt to account for 

 their origin. 



An inspection of the figures quoted above from Jefferson^ or, better, of 

 the diagram (figure 5) to which he refers, shows one of the aberrant 

 features which sometimes obscure the regularity of spacing. This con- 

 sists of a compound cusp having three apices, between which are included 

 the abnormally small spaces of 6 and 7 feet respectively. Unless the 

 conditions of wave activity changed, the compound cusp would eventually 

 be fashioned into two normal cusps by the destruction of the central pex, 

 possibly accompanied by an enlargement of the space thus gained until it 

 more nearly corresponded in size to neighboring spaces. Thus the only 

 pronounced irregularity in Jefferson's figures represents a temporary 

 failure of the waves to complete the approximate equalization of inter- 

 cusp spaces. The following figures illustrate the spacing as measured on 

 four different beaches. Successive figures denote the number of paces to 

 successive cusps. 



I 



31-35-35-34-33 



II 



12-12-ll-10-12-13-10-10-n-9*-8^-13-15-ll-10-10-12 



III 



9-9-7-7-8-9-10-7--9*-10-8-6*-7*-7-6-6-6-6^-5^-5*-5-7 



IV V 



28-28-28-27 11-11-11-5 f 



VI VII 



17-17-15-9-17-15 14-19-17-18-13-12-12-15-15 



VIII IX X 



27-27-30 13-13-19 6-7-7-8-6 



I. Sand cusps on beach south of Dyker Heights, Brooklyn. A. W. Grabau and 



D. W. Johnson. 

 II and III. Older and later cusps observed at same time on Winthrop Beach. 



D. W. Johnson. 

 IV and V. Older and later cusps observed at same time on Pond Beach, Na- 



hant. M. E. Blodgett. 

 VI and VII. Cusps on small beach near Nahant; observed at different times. 



M. E. Blodgett. 

 VIII, IX, and X. Three sets of cusps observed on upper, middle, and lower 



parts of Pond Beach, at same time. D. W. Johnson. The number of 



measurements i«5 small, but represents the degree of variation in spacing. 



* Part of compound cusp. f Imperfectly developed. 



