FOSSIL GEOLOGY OF UNITED STATES. 301 



A portion of the same Marble polished. 



This marble is extensively employed in England 

 for sideboards, tables, and ornaments. What are 

 called the pulley, or screw stones, are merely the 

 sharp impressions of encrinital stems, forming solid 

 cylinders, marked with annular risings and depres- 

 sions. The transition limestone (and the slate for- 

 mation in England) are the grand repository of these 

 remains, upward of 50 species, belonging to 25 gen- 

 era, having been collected from these ancient de- 

 posites. They are almost unknown in the upper 

 secondary and tertiary formations ; a fact which, in 

 the opinion of geologists, indicates some remarka- 

 ble change in the physical condition of the waters 

 during these later periods, inimical to the great de- 

 velopment of the crinoidea, which existed in the 

 oceans of the transition era 



