THE MUSEUM. 



359 



terruption to navigation, that the gov- 

 ernment thought it advisable to con- 

 struct a canal to obviate the difficulty. 

 The rock is limestone, of a very hard 

 and durable nature. Some of it actu- 

 ally passing the border line into mar- 

 ble. 



Like other Pahtozoic rocks, these 

 strata had their birth in an open sea. 

 And they are mainly formed of the 

 remains of animal life, every one of 

 the five sub-kingdoms of which is rep- 

 resented in these rock strata that un- 

 derlie the city. Protozoa are not 

 largely represented by fossil remains. 

 Two species of sponge are known. 

 But of Radiates there is a better show- 

 ing; for ZapJircntcs is a rather com- 

 mon coral, and Crinoid remains, in the 

 way of detached broken arms and 

 stems, are plenty. The perfect fossil 

 is not easily obtained, but sometimes 

 many species are found crowded to- 

 gether in a small space, so that from 

 an area of ten feet square, 200 have 

 been procured, seeming to show either 

 a gregarious habit in the animal, or a 

 retreat into protected places for safety, 

 or a more congenial place of growth 

 because of currents then existing, or in 

 the amount of food wafted to those 

 favored localities. 



The following genera are most large- 

 ly represented viz: Actinocriiius, 

 Forbcsioerimis, Cyathocriinis, Platy- 

 criniLS and Agariocriiius. 



The Molluscan remains are well 

 represented by Brachiopods, of which 

 the genus Spirifer is, perhaps most 

 abundant, yielding seven or eight spe- 

 cies. Also Or this is quite abundant. 

 The Gasterpods make a poor showing. 



Of Vertebrates there are the remains 

 of many Selacian, or Cartilage fish, in 



the way of teeth and fin spines. So 

 that taking the number of these found 

 in this locality, seems rather to add 

 another doubt whether the sub-carbon- 

 iferous is not really better entitled to 

 be called the "Age of Fish" than the 

 Devonian, as that has usually been 

 styled. 



The upper beds of the Keokuk for- 

 mation properly are famed for their 

 Geodes. I think a true explanation 

 of how these have been formed is yet 

 wanting. It is easy enough to call 

 them ''Genus Biopella,'" but how 

 formed this interior lining of crystals, 

 of quartz, or calcite or both, as the 

 case may be, sometimes almost to the 

 obliteration of the internal space. 

 The Geode sometimes contains liquid, 

 but if all had originally been filled 

 with the most concentrated solution of 

 silica or lime carbonate, it would, by 

 evaporation, be entirely insufficient in 

 quantity, to yield the interior lining of 

 crystals. After the crust of the geode 

 was formed, being very generally im- 

 pervious to water, no solution holding 

 material could find its way inside. 

 Besides, when at home in their native 

 bed, the matrix rock is a roft magne- 

 sia limestone, from which the desired 

 material for the included crystals, we 

 would rather suspect to be Dolomite 

 or Calcite and not Quartz, which is 

 most common. When the matrix 



rock in this vicinity approaches to a 

 purer limestone in constitution, the 

 geode seems to vanish — cavities in the 

 rocks, often beautifully lined with 

 crystals, taking its place, but the ex- 

 terior shell is obliterated. It has ad- 

 hered to, and become a part of the 

 surrounding rock, and is not separa- 

 ble from it and the cavity very often 



