No. 2] SNAILS OF THE GENUS 10— ADAMS. 43 



ness in relief is produced, similar to that of West Virginia. With continued erosion the divides 

 tend to become lowered and less in number, and the drainage more concentrated and adjusted 

 to a condition of relative equilibrium. In this manner a peneplained surface is again acquired, 

 and a cycle of erosion is completed or developed. 



3. Let us now consider the influence of a large uplift and foldiag of the strata parallel to 

 the axis of the former main divide and intermediate between this divide and the coast, or the 

 major axial stream. 



If we assume that such an uplift is gradual, it will tend to retard or pond the minor streams 

 above it and form a divide along the axial liae of the uplift. The small stream will flow down 

 such a slope and its head will tend to migrate up the slope toward the crest of the uplift. A 

 trunk or axial stream will tend to form in the depression (Campbell, '96, p. 658) and parallel 

 to the axial divide. Major through-flowing streams crossiag such an uplift on account of their 

 erosive power and volume may draia the streams ia the axes of the depressions. These same 

 processes and conditions will be repeated as frequently as there are distiact folds or trough 

 valleys in the uplifted and folded belt. 



It is also possible to thiak of the various degrees or transitional stages between this con- 

 dition and one of a sudden uplift, when draiaage is ponded and turned iato axial depressions 

 formed by the uplift and its foldiags. 



If, however, the upturned rocks are composed of beds of relatively soft and hard rocks, 

 erosion upon them will be correspondiagly varied and complex in its details. Although all the 

 surface rock may be rather uniform and resistant, yet the uplift will hasten erosion and enable 

 it to penetrate to the softer rocks which, once exposed, will erode with relative rapidity and 

 become reduced as previously outlined, so that such an uplifted area will be peneplained some- 

 what similar to that before the uplift. 



/. The faunal criterion of drainage changes. — The value of faunal evidence ia studying 

 stream development presents a problem over which there has been some confusion and discus- 

 sion. Johnson ('05b) has considered this criterion and concludes that faunal evidence can 

 only be suggestive or secondary and is not positive proof of draiaage changes. It is well that 

 this distiaction has been emphasized, as the position seems a valid one. The faunal argument 

 must not be too general or it loses the iafluence of the local environment, and, further, it must 

 harmonize with the ecology of the particular animals involved rather than with the group in 

 general. In other words, each case must be tested on its own merits and harmonize with the 

 local conditions and the special ecology of the animals. Of course there are degrees of value 

 in this secondary or suggestive evidence of stream changes, and it is particularly to this subject 

 that most attention should be directed in the future. Throughout the present study of lo the 

 suggestive value has been recognized and a special effort has been made to utilize the physical 

 history of the streams. The general faunal evidence has not been considered. In the past 

 there has been a tendency toward placing too much emphasis upon ''accidental dispersal" 

 (White, '83, p. 482), and this idea has retarded a just valuation of the real importance of a 

 knowledge of stream histories which may be developed by the convergence of all liaes of evidence. 



The facts of distribution of lo can. not be a positive proof of draiaage changes, because 

 these snails were apparently used as food by the Indians; at least these shells are abundant 

 about the sites of old Indian camps and mounds, to which they had been carried. Shells thus 

 transported by the Indians might be carried from one drainage line to another, and yet even 

 such dispersal would not seem to have been very general, because of the amount of individuality 

 now shown ia the shells of different streams. Of course at the confluence of streams there could 

 easily be such an iaterchaage of shells by the Indians ; and yet this would only be hasteniag a 

 process which would ia all probability take place, though at a much slower rate, by the activities 

 of the snails themselves. Portages would possibly seem to offer the most favorable situations 

 for transportation and mingliag of these shells, but draiaage modificatioas may also be iavolved 

 in such localities. I have not been able to recognize the influence of this factor. 



