206 GENERAL GEOLOGY. 



The upper division furnishes excellent common quarry rock 

 wherever it is exposed, and from some of the layers suitable 

 blocks are obtained for dressing into the various forms needed 

 in the better kind of buildings. The rock of this division is 

 also strong and endures exposure well. That of both divi- 

 sions has been largely used for macadamizing the streets of 

 Burlington, to which is sometimes added the flinty masses of 

 the silicious beds, the only purpose for which the latter are of 

 any economic value. The upper division is of the greatest 

 economic value, the material being a little better than that of 

 the lower, greater in amount and occupies a greater surface 

 area. 



The color of some portions of the limestone of the upper 

 division is so nearly white and its texture somewhat crysta- 

 line, the purer pieces resemble marble, but the want of 

 uniformity in its texture and the presence of silicious lumps 

 and particles in it will prevent its successful use for any orna- 

 mental purpose. 



Fossils. The Burlington limestone received its name from 

 the city where its peculiar characteristics are best shown, and 

 have been most studied. The formation has in turn caused 

 the name of the city to be spoken among men, who otherwise 

 would have hardly been aware of its existence; for the great 

 abundance and variety of its characteristic fossils — crinoids — 

 have justly attracted the attention of geologists and natural- 

 ists in all parts of the enlightened world. Although the area 

 occupied by the outcrop of this formation in Iowa is compara- 

 tively so small, yet the fossil remains it has afforded are of 

 the most remarkable character and profusion. The abun- 

 dance and variety of the crinoidal remains of both divisions 

 constitute indeed the most distinctive palseontological feature 

 of the formation, but were this characteristic absent or less 

 conspicuous, it would still have a distinguishing palseonto- 

 logical feature in its fish remains. 



Vertebrates. The only remains of vertebrates which the 

 formation has thus far afforded, are those of fishes. The* 

 greater part of these are the teeth and spines of Selachians— 



