266 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 22, 



the marine moUusca are found ; but we here suddenly reach a 

 totally different formation. A stiff tenacious blue clay overlies the 

 shelly sands, and a corresponding change in the animal remains 

 shows how the conditions of life were modi Bed by those events 

 which produced this change in the nature of the subaqueous deposits. 

 These marly beds are considered by the German geologists to repre- 

 sent the lower brown-coal formation of the Wetterau, and are called 

 by some the Cyrenen-Mergel or Cyrena-marl, from the abundance of 

 Cyrena subarata found in it ; with this are associated several species 

 of Cerithium, and a few forms of littoral Gasteropods. 



This formation occupies a considerable area in that portion of the 

 Mayence basin which lies to the westward of the Rhine (see tig. 8), 

 appearing in many of the bottoms and hollows, and even constituting 

 low undulating hills. Here it has generally a very slight dip or in- 

 clination to the eastward ; but on the opposite or eastern side of the 

 Rhine the dip is reversed, and it only appears in a very few localities, 

 and at a very low level. Between Hochheim and Florsheim it occurs 

 on the bank or in the bed of the Maine, where it contains Cerithium 

 margaritaceum of a large size. Here a slight rise towards the east 

 appears to be indicated ; for in the section taken near Hanau, fig. 9, 

 it forms the basis of the whole tertiary series, at a slight elevation 

 above the bed of the river. This section, which is the most eastern 

 point I had an opportunity of examining, extends from the lower 

 to the upper blue clay, and will be subsequently described. (Hanau 

 is about 30 miles N.JE. by E. of Florsheim.) 



The principal localities where this blue clay has come under my 

 notice, in addition to those already incidentally mentioned, are the 

 Zeil Stiick near Weinheim, where many fossils have been found, 

 and the Sommerberg, opposite Weinheim ; it also occurs abun- 

 dantly on the road between Alzey and Mayence, near the village of 

 Ensheim, where Mr. Austen and myself found Cyrena subarata and 

 a small Cerithium ; and again, on another road to Mayence, near 

 Partenheim, where I obtained numerous Cerithia on a former occa- 

 sion. The septarian clay near Diirckheim also evidently belongs to 

 this formation, while to the N.W. it extends at intervals from Alzey 

 to Kreutznach. 



III. Cerithium Limestone. (Nos. 3 and 4 of the German list, p. 260.) 



I omit the No. 3 of the German table, which is evidently only 

 the lower portion of this limestone, into which considerable numbers 

 of land-shells have in particular spots been drifted. The beds of 

 which this division of the series consists are generally hard, compact, 

 and thick-bedded. In many places, indeed, it is difficult to detect 

 evidence of distinct stratification. They are also generally more ex- 

 tensively developed on the left bank of the Rhine than on the right, 

 where the next succeeding bed, the Littorinella-limestone, is more 

 abundant. It is however difiicult, if not impossible, to draw any 

 distinct line between them, except in so far as their fossil contents 

 differ, and that the Littorinella-limestone is somewhat less thickly 

 bedded, and is often interstratified vaih. beds of marl or clay. The 



