ADAMS COUNTY, 53 



perfectly their original form and are frequently of a reddish pink color from 

 the silicious matter, which has replaced the carbonate of lime in the original 

 coral. These corals belong to the genus Lithostrotion, and are known as the 

 L. canadense and L. proliferum, and the former species, which usually occurs in 

 massive forms, is popularly known as " petrified honey comb," from the polygo- 

 nal form of the numerous calyces of which it is composed. 



In the magnesian and shaly beds of this group, fossils are usually quite 

 abundant, and among the most striking forms we may mention the screw shaped 

 fossil, known as the Archimedes, the axis of a peculiar form of Bryozoa. The 

 largest form of this interesting genus, the A. Wortheni, of Hall, is found 

 abundantly through the shaly beds of this group, and some of the largest 

 specimens attain to a foot or more in length. Various other forms of Bryozoa 

 also abound in this rock, and at some localities, the magnesian beds of this 

 group appear to be in good part composed of the delicate, reticulated remains 

 of this class of organic forms. Marine shells are also abundant in the same 

 beds, among which are Spirifer lateralis, S. sub-sequalis, Rhynchonella mutata, 

 R. subcuneata, Retzia Verneuiliana, Orthis dubia, Terebratula hastata, Platyce- 

 ras, acutirostris, and Productus Altonensis. A knowledge of these species will 

 enable the observer to identify this formation wherever it may appear, as some 

 of them have a wide geographical range, especially the Lithostrotion canadense, 

 which is known to range from Illinois to Alabama, and on a recent visit to Utah, 



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we found it embedded in the highly metapjbewa limestones of the Wahsatch moun- 

 tains, within twenty miles of Salt Lake City. Hence we may understand the 

 great value of fossils to the geological observer, as they enable him to establish 

 the identity of strata at widely separated points, where the lithological charac- 

 ters of the beds are completely changed, and where it would be impossible to 

 trace the continuity of the strata. 



Keokuk Group. This group immediately underlies the limestone just des- 

 cribed, and usually appears in two well marked divisions. The upper one 

 consists of bluish gray or grayish brown calcareo-argillaceous shales, and shaly 

 limestones, enclosing silicious geodes of various sizes, some of them a foot or 

 more in diameter, a part of which are solid spheres of crystalline quartz, cov- 

 ered externally with a thin coating of chalcedony, while others are hollow, and 

 have their inner surfaces covered with beautiful crystals of quartz, calcite, or 

 dolomite, or with the mammillary forms of chalcedony. Crystals of arragonite, 

 iron pyrites and zinc blende are are also occasionally found in these silicious 

 geodes, and the finest cabinet specimens of the crystallized minerals above 

 mentioned to be found in this State, are obtained from this bed. The shales 

 and shaly limestones in which the geodes were originally embedded, yields 

 readily to the influence of frost and moisture, and the silicious geodes are 

 readily weathered out and may be found in great numbers in the beds of the 



