108 THE NAUTILUS. 



ing the physical conditions of the time of the Loess deposit strongly 

 recommend themselves to us. Not much evidence of colder climate 

 than now prevails over the same district can be derived from the 

 fossils, which differ but slightly, when at all, from recent specimens 

 from the same locality. Mr. Shimek concludes that — 



The climate was comparatively uniform during the entire period, if we may judge from the 

 fossils, for the same forms are often found from the very base, just over the drift, to the upper- 

 most portions of deposits one hundred feet or more in thickness. No difference is noticeable be- 

 tween specimens of the same species from the lowermost and the uppermost portions of the de- 

 posits, — a fact which warrants us in concluding that climatic changes during the deposition of the 

 Loess could not have been very great. The fact may here again be emphasized that while a de- 

 pauperation is noticeable in some of the species, it is far from sufficient to indicate a temperature 

 such as would be necessary to retain lakes and streams within shores of ice during the summer, 

 and that consequently the Loess was not deposited during a glacial climate, but at a time when the 

 temperature had moderated sufficiently to enable many land-shells, now restricted to or abundant 

 in middle latitudes, to flourish in considerable numbers. 



The great predominance of strictly terrestrial species ot molluscs in the Loess indicate that 

 during its deposition large areas of land-surface were exposed, at least during the greater part of 

 the summer, upon which these molluscs lived and multiplied under conditions which exist now in 

 the habitats of their modern representatives. 



For the belief that the fossils as now fonnd are not far removed from the localities in which they 

 lived and developed, a number of valid reasons exist, the most striking of which are the following: 



1. Their usually perfect preservation Such delicate shells as many of those under con- 

 sideration could not be transported far by turbulent streams without being broken. 



2. Their distribution , both vertically and horizontally . In many exposures of the Loess the 

 species which to-day have the habit of remaining in considerable numbers in very restricted 

 localities, as Helicina occulta., Patula strigosa cooperi, Mesodon multilineata, the Limneece, etc., 

 are likewise similarly restricted in their horizontal distribution to very narrow "pockets," 

 though the same species may often be traced vertically for many feet, as though the shells had 

 gradually accumulated through many generations. Again other species are more generally and 

 more uniformly distributed in a manner which recalls their present habits, and which indicates 

 that they have not drifted into the places in which we find them to-day,— at least not sufficiently 

 far to disturb the arrangement with reference to each other, which we may observe in the living 

 specimens. 



The writer's conception of the climate and of the origin of the Loess, based largely upon a care- 

 ful and extended study of its fossils, may be briefly summarized as follows : 



I. The summers during the formation of this deposit were comparatively warm, and the 

 glaciers had already retreated far to the north when the deposition commenced. 



II. During at least a part of the summers a large portion of the area now covered by the Loess 

 was elevated above the surface of water, as indicated by the presence of the predominating land 

 shells. 



III. These shells, too, indicate that the surface was not entirely unlike our present prairies in 

 Iowa and Nebraska, though perhaps more moist, and more nearly level. 



VI. The presence of shells of the genera /,z>««^a, /'/ij'ja, Planorbis, z.nA Pisidium, which 

 are principally pond species, indicates that over this prairie surface were scattered numerous 

 ponds and that it was traversed by quiet, sluggish streams. 



V. The distribution of the shells as well as the homogeneity and fineness of the material form- 

 ing the deposit indicate that the deposition was unaccompanied by violent disturbances, but 

 that it took place quietly, and very slowly during a long period. 



VI. The deposit itself was probably partly formed from sediment carried over portions of the 

 surface by quiet overflows of the sluggish streams which had not yet deeply cut their channels. 

 The numerous ponds, however, fed by the drainage in their immediate vicinity, were also receiv- 

 ing with this the finer material gathered from the glacial drift surrounding them. This material 

 being like that gathered by the streams, would form similar deposits, each pond or swamp form- 

 ing in this way a bed of Loess. The changes in the level of water in the ponds would produce a 

 change in the extent of the muddy flats along their shores, thus facilitating the distribution of the 

 terrestrial shells. These ponds and streams, by shifting about through the combined influence, 

 of floods and diouths, extended the distribution of the sediment, and subsequent erosion completed 

 the work necessary to produce the present topography. 



