No. 136.] 53 



western extension of these strata in Wisconsin, this mineral is 

 perceptible only in very small quantity. 



Some very curious and interesting markings have been found 

 on the surfaces of sandy layers of this rock in Oneida and Her- 

 kimer counties ; appearing to be the tracks or trails of shellfish 

 or some other humble form of life, which crawled over these 

 sands, perhaps while exposed just above the water level by the f^, 



reflux of the tide, which on its return washed over them another 

 film of sediment which preserved them through all subsequent 

 changes until now. Specimens of these trails are in the cases, 

 and are figured in the second volume of the Palseontology. It is ' 

 one of the strangest of 'facts, that what we consider the most 

 striking symbol of evanescence, a track upon the tidewashed 

 sand, should thus become an imperishable record. (Other • 

 instances of the same nature are known, one of which is the 

 existence in strata of the Connecticut sandstone, a rock of far 

 more modern age, of tracks of wading birds like snipes or herons, 

 some of them being of great size ; doubtless imprinted on the 

 beach laid bare by the tide, or in shallow waters, where this rock 

 then existed as a soft bed of sand. Large slabs of this rock, 

 with such impressions, are in the upper story of the Museum.) 



The fossils of the Clinton group are quite numerous, but not 

 generall}^ so marked as to be described with interest in a general 

 sketch like the present. Many of them much resemble those of 

 the succeeding rocks- the most characteristic being a large 

 bivalve brachiopodous shell, the Pentamerus oblongus, common 

 below* Rochester. It is oval and somewhat flattened inform: 

 the internal structure, which can be seen in some specimens, is 

 peculiar, -but not explainable without plates. This shell is a 

 peculiarly interesting fossil, as it is known to spread, in strata 

 of similar age, from Europe to the west of the Mississippi ; a 

 marked instance of the wide extension of a single form in the 

 ancient ocean. Its general form is shown by the figures in the 

 woodcut 14a. 



To the Clinton group succeeds the 



NIAGARA GROUP. 



The observer, standing on the upper Suspension Bridge, sees in 

 the precipice, above the Clinton limestone, a sloping bank of soft 

 grey shale about eighty feet thick, above which follows a thick 



