OP PLANORBIS AT STEINHEIM. 



41 



If the rocks to the east of the Heidenheim road had contained any Planorbis, Lymneus 

 or Gillia it might have been said that the Cloister ridge and these rocks together, con- 

 taine<l a fauna precisely intermediate in character between the Buro-stall and the lower 

 strata of the Pit Deposits. The presence of HyaUna subnitens and Vitrlna suevica, how- 

 ever, both fossils of the Sylvana Limestone, may be perhaps explained in the same way as 

 the reappearance of PI "X:!^"' in formation m of the Cloister Pit, namely by colonization 

 from some neighboring part of the lake where they had continued to exist. With these 

 exceptions, the fauna of these Heidenheim road rocks certainly presents a somewhat 

 remarkable character and contains an association of species which appear to follow on very 

 naturally after those of the Lower Tier, and fill the gap between these and those of the 

 lower strata ol the Pit Deposits. This rock is very similar, as remarked by Quenstedt, to 

 the limestones of the Pit Period, and at first sight seems to be filled by the normal vari- 

 eties of PI. discoideus, to the exclusion of other forms, and has altogether a more recent 

 aspect than the rocks of the Cloister Ridge. 



The forms, however, do not indicate a geological transition, because it is in the Lower 

 Tier that the pi< 'dominance of discoideus occurs and not in the Upper. The Upper 

 Tier appears to lie upon that here called the Lower Tier, though with regard to this the 

 data are not such as would render this view unquestionable. The division between the two 

 Tiers is uncertain and not sharply mari; jd off by any line of stratification. It is 

 possible that what now is the Lower Tier does not underhe the Upper Tier, but may have 

 once covered the summit of the hill and been superimposed upon the rocks now exposed 

 by denudation above. In this case the anomaly of the prevalence of " discolLeiis" in 

 the lower bed instead of the higher, would be done away with. 



Whatever results may flow from future investigations upon the relations of the rocks 

 of the circular valley, it does not seem to me probable that the bearing of these facts 

 upon the origin of the fauna of the Pits can ever be materially altered. The rocks 

 contain a fauna, which is probably older than that of the Pits, and shows that the forms 

 which here and there appear suddenly at the lowest levels in the Pits had their origin in 

 a former period of which these rocks are the imperfect remnants. The richness and the 

 sudden development of the forms of the lowest stratum of the Pits at the two places 

 examined by me require an explanation, and that given by Hilgendorf does not seem 

 to me wholly satisfactory. Granting that the spots examined by me were nearer to the 

 shore line, and his farther out in deeper parts of the lake, and that he found only 

 PL Steinheimensis and PI. parvus in the lowest stratum of the Pit deposits, the 

 fact remains that my explorations reached the bottom of the deposits in two places. 

 If the fauna I found was not contemporaneous with that which he found, the shores 

 must have been very steep, and have prevented the burying up of the shore-line faunas 

 until all the deeper parts of the lake were filled. This, however, is a difficult matter 

 to prove, in view of the fact that the strata have been more or less elevated since they 

 were deposited. 



The Cloister Ridge has suffered greatly from denudation, and standing on the summit 

 alongside of one of the pillars or knolls, which still remain, the conclusion, that the 

 entire valley or basin must have been originally covered by rocks of a similar kind, 

 which have been, however, almost wholly removed, seems to be well supported. 



