The Saint Peter Sandstone. 69 



Peter Sandstone, near Saint Paul, Minnesota. A notice of 

 the same has been published (29). Fossils can be found still 

 at the same localities. At Dayton's Bluff, the "Carver^s 

 Cave" of early writers, fossils were found between 60 to 80 

 feet below the overhanging Trenton, or Buff Limestone. A 

 second exposure at Highland Park two miles south of the 

 first is in strata lower than any at Dayton's Bluff, and here 

 also fossils were found. At a third exposure, which is near 

 South Saint Paul, there were found by Professor C. V/. Hall 

 and by myself a large number of species, in strata that are 

 probably intermediate between those of Highland Park and 

 Dayton's Bluff. 



Condition of Preservation. 



The fossils are found as casts of shells that have them- 

 selves been entirely dissolved without leaving even a stain 

 of color or a trace of calcium carbonate in the sand. The 

 cavities left by the shells are closed up by a consolidation of 

 the sand in some manner so that generally little more than 

 smooth cleavage planes remain to define the fossils. In 

 the strata that are referred to the upper half of the forma- 

 tion, the fossils were nearly always ree from distortion. In 

 the lower half on the contrary, lew are not distorted. Here 

 too there were found fossils which are the moulds only, of 

 the casts of shells. The moulds were full of loose sand, 

 which when removed simply by blowing and shaking, left 

 smooth cavities in exact form ot the interiors of shells them- 

 selves. The loose sand in the moulds was probably once 

 embedded in the casts of shells. The process of fossilization 

 must have been nearly as follows: A shell produced a cast, 

 and was later dissolved away. The inner cast of the shell 

 remained firm while the sand around it was disturbed and 

 pressed around the cast taking the mould of it. The cast 

 itself was then reduced by removal of the calcium carbonate 

 to loose sand, the mould, however, remaining firm because 

 already consisting only of sand. 



Similar phenomena are to be observed in dolomitic 

 limestone. The fossilization itself is not strange, but it re- 

 veals to us perhaps the reason that Saint Peter sandstone is 

 so generally unfossiliferous, viz.: Because the fossils have 



