OF PLANORBIS AT STEINHEIM. 65 



adopt it in the table as a fixed level through which I have drawn a dotted line. Depart- 

 ing from this in any section, it becomes possible to synchronize the different formations. 



If we assume that the usual sedimentary matter held in suspension by the waters of the 

 lake was clay, and that these waters were saturated with lime, we have an explanation of 

 the rapid formation by deposition of the plates of limestone. This being the necessary 

 consequence of the undisturbed action of the water, we should have the dense limestone 

 layers deposited on the bottom wherever the currents were not disturbing it and spreading 

 out the coarser sand derived either from the Cloister Ridge rocks or the adjacent surfaces 

 of the Jura. 



The constant shiftings of the local currents, due either to the obstacles they themselves 

 had built up or other disturbing causes, would produce this aspect of regularity in each 

 section of the layers, as well as the want of correspondence in the synchronous deposits of 

 even adjoining sections. 



The widely distributed formations could never be limestones, but might occasionally be 

 composed of materials derived during floods from the surrounding country, that is of clay 

 or sand. A glance at the sections will show that this is the case since e is of shell-sand, 

 m of clay, and none of the limestone tables are continuous. The general changes, the 

 predominance of shell-sand during the Trochiformis period, and of clay during the Oxys- 

 tomus period, would also seem to be accounted for by a greater or less prevalence during a 

 certain number of successive seasons of similar deposits, due to changes in the localities 

 from Avhich the greater part of the drainage was derived, or to other local causes. 



In this condition the Steinheim deposits of the Pits assume the aspects which might be 

 expected to arise in a land-locked lake with a central island. The deposits would be 

 formed in some places from the debris washed off of the rapidly disintegrating surface of 

 the island, and in others, even in close approximation, the ordinary formation of limestone 

 or the precipitation of fine sandy material, or flocculent clay, might take place in quieter 

 water, or farther from the shore. Any of our inland lakes present similar conditions 

 wherever local streams empty into them. During heavy rains as at dffierent seasons of 

 the year, the debris of the beaches and bottom is subject to noticeable variation within 

 very short ranges. 



Another fact in this direction indicates also that the amount of time represented by 

 the Steinheim Pit Deposits must have been very limited. Strata slowly formed are 

 marked off in exceedingly thin layers, since but a small amount of fine sediment is held 

 in suspension by the water, and slowly deposited during a given period of time. The 

 thicker layers result from a larger amount of sediment which has been held in suspension 

 and falls with greater rapidity. This accounts for the finer bedding of the clays, 

 fine grained sandstones, limestones and so on, as contrasted with the coarser rocks and 

 rubble. 



The strata and sometimes entire thick beds of shell-sand bear no marks of stratification, 

 and must, therefore, have been built up by continuous and rapid deposition. The 

 cleiy layers are of various degrees of thickness, but usually an inch or more, 

 and very rarely of paper-like thinness, and this is true also of the limestones. The 

 fish-layers of formation c are particularly instructive in this respect. The fishes being 

 necessarily very destructible, testify to several things : first the rapid deposition of the 



