T128] RIPLEY CROUP. 83 



Very perfect impressions of fish have been found in several localities, mostly 

 in digging cisterns ; as at Camargo, Palo Alto, and Okalona. I have never been 

 so fortunate as to obtain even a fragment of one. 



Irregular, rounded nodules of iron pyrites, of a radiated structure, called 

 "Sulphur Balls," are common throughout the Eotten Limestone, and sometimes 

 cause considerable difficulty in boring wells, on account of their hardness, and 

 tendency to divert the auger from the vertical. 



IV. THE PIPLEY GROUP. 



128. The surface of the territory occupied by this, the upper- 

 most stage of the Cretaceous in Mississippi, is generally hilly, and 

 to a great extent, thickly covered with the strata of the Orange 

 Sand, which have filled up the gaps occasioned by fracture or de- 

 nudation in the ridges formed by the upheaved strata of the group. 

 Small prairie spots are met with in many localities, but usually on 

 or around isolated hilltops or ridges, where some soft calcareous 

 stratum has approached the surface. On these "bald prairie hill- 

 tops," we often find in abundance the Exogyra costata and Gry- 

 phaea mutabilis (G. convexa is less common), but associated with 

 them are always nuclei, at least, of fossils characteristic of this 

 group. 



There are two materials especially, which in their various modi- 

 fications, compose the strata of this group, viz : hard crystalline 

 limestone, more or less sandy and glauconitic, which forms the 

 highest strata ; and bluish micaceous marls, more or less sandy 

 and often interstratified with subordinate ledges of sandy limestone, 

 which latter become less and less frequent as we descend in the 

 series towards the strata forming a pala^ontological as well as 

 lithological transition into the Rotten Limestone. 



In the uppermost, hard varieties of the limestone, the substance of the shell 

 of fossils is generally replaced by crystallized, transparent calcareous spar, 

 which often forms specimens of great beauty and perfection. Lower down we 

 often find hard nuclei, while the substance of the shell is soft and friable crys- 

 talline matter, or at times, is altogether wanting, so as to leave the nucleus 

 either loose, or fixed at some point, standing free within the hollow space. In 

 the marls, on the contrary, the preservation of the substance of the shells is 

 very perfect, since it has not been replaced by any extraneous matter, but sim- 

 ply rendered friable by the decay of the animal glue. It generally exhibits, 

 therefore, not only the finest details of structure, both external and internal, 

 but even the iridescence of the mother-of-pearl is often beautifully preserved, 

 so that the observer at first finds it difficult to convince himself that the beds 



