OF PLANORBIS AT STEINHEIM. 43 



the forms which are so abundant in the Cloister Pit, which is surrounded by the^r, 

 appears highly improbable. 



In fact the enclosed pieces of rock, the general composition and inclination of the 

 strata, and the fossils of the lower Pit Deposits, all seem to indicate for them an origin 

 later in time, and a partial derivation of the material from the Cloister Ridge rocks. 



The uppermost Pit Deposit found by me on the hill immediately above the Old Pit, and 

 containing Lymnea socialis and described as a rubble derived from the Cloister Fddge 

 rocks, also indicates a similar origin. 



The structure of the Cloister Ridge rocks indicates a contemporaneous origin with the 

 lower rocks on the Burgstall, as noticed by Professor Fraas, but the fossils are not identical. 

 No exposure of any rock under the Lower Tier was seen, although diligently sought for, 

 and I was disappointed in this last hope of obtaining positive proof of identity between 

 the rocks on the Burgstall and those on the Cloister Ridge. The impression made by the 

 fossils and the rock structure was such that under ordinary circumstances, and in a less 

 important locality, I should hardly, however, have hesitated to consider the Cloister 

 Ridge rocks as belonging to a somewhat later, if not contemporaneous part of the same 

 general deposit as those of the Burgstall. 



The Pit Deposits do not extend far out into the valley, but are limited to remnants 

 clinging to the sides of the central hill. The underlying clay, the White Jura /S, has 

 been described as occurring wherever wells or cellars have been dug in the village, and 

 was found by Dr. Hilgendorf and the author in the Old Pit at the base of the deposits. 

 According to Dr. Hilgendorf, however, the White Jura forms the base of the East Pit, 

 and the Opalinus Clay, a much older formation of the Brown Jura, the base of the Cloister 

 Pit. This last fact shows how great the denudation must have been which took place 

 before the Pit Deposits began to be laid down on the south side of the hill. 



It is very evident also from the singular want of exact agreement between the 

 layers of adjoining localities, as for example in Sections 7 and 8, representing two nearly 

 opposite sides of the New Pit, that the physical conditions varied considerably within 

 a few yards, and that while limestone or clay was accumulating in one spot, sand 

 was being laid down in another immediately adjoining, and the same for greater 

 distances, and in a larger sense. As, for instance, the sands and hmestones of the Cloister 

 Pit are in direct contrast with the great prevalence of clay in the corresponding parts 

 of the New Pit and East Pit. There must have been, therefore, very considerable 

 variation in the state of the water in these different spots, though in such close proximity 

 to each other. Nevertheless, with all this local variation, there is a regularity in 

 any one section in the succession of limestones, clays, and sand, which strikes the 

 observer at first as absolutely uniform. Thus though no great amount of uniformity 

 exists such as would enable us to synchronize the strata with exactitude in different local- 

 ities on the same level, there is great uniformity in the alternation of limestone and shell 

 sand, or clay and shell sand, and this may be seen to be the governing fact in any one of 

 the sections. These minor local differences of structure can not be used to explain the 

 greater differences of the Cloister Ridge rocks, since in no case are the partings of lime- 

 stone formed " in situ" of a similar structure. They are everywhere a shell limestone of 

 greater or less fineness, but never vesicular, or similar to the limestones of Cloister Ridge 

 rocks except in the lowest parts of the Pit Deposits. 



