McOee and Call — Loss of Des Moines, Iowa. 215 



The desirability of tracing the superior and inferior surfaces 

 of the loss toward the periphery of the once continuous plateau 

 on which it is found, and to their termini, was fully realized ; 

 but it was found impracticable to do so. Drift materials were 

 however found beneath the loss in section 7 ; and this is known 

 to be the normal sequence of these deposits. Moreover, the 

 superincumbent drift-sheet has been traced uninterruptedly, 

 in different directions, to localities in which no loss occurs 

 beneath it. The stratigraphical relations of the loss of this 

 as shown in the accompanying 



Recapitulating the salient stratigraphical features of the 

 region under consideration, it appears, (1) That the loss is 

 -confined to elevated plateaus; (2) That its upper portion is 

 broken up, contorted and interstratified and commingled with 

 rift; and (3) That the whole is overlain by unmodified 

 glacial drift. 



The first of these features is consonant with the phenomena 

 observed by the senior author in northeastern Iowa, where the 

 !'">>.- similarly affects the highest summits and divides; but the 

 •others are unknown elsewhere. 



III. 



The accompanying synoptical faunal tables will afford the 

 means of determining the relative abundance and variety of 

 molluscan life during the formation of the deposits studied as 

 compared with the present. The twenty-four species listed in 

 table I are unequally distributed among fifteen subgenera, and 

 these again unequally among five families. The Helicidai, 

 Pupadce and Helicinidce* are terrestrial groups, the Limmetdce 

 a fresh-water family, while the remaining one, together with 

 the subgenus Succinea of the Helicida may be quite properly 

 considered semi-aquatic, f Table II contains nearly twice the 

 number of forms found in table 1, the 



Nevertheless its optin 



