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 encrinitai 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 th^ 
oceans of the transition era. 
