TRANSITION LIMESTONE. 83 



occurs in beds alternating with slate, gray wacke, and 

 coarse sandstone, and sometimes forms mountain 

 masses. It contains but few petrifactions. This 

 rock abounds in natural caverns, which often seem 

 to have been formed by the agency of water peico- 

 lating through natural fissures, and in the lapse of 

 ages excavating the softer or more broken parts of 

 the rock. Transition limestone abounds in metals ; 

 the principal ores are those of lead and zinc, which 

 commonly occur in veins. It contains also organic 

 remains, such as encrinites, madrepores, and corallites. 

 In England and Wales alone, 553 specimens of or- 

 ganic fossils have been discovered in what is called, 

 by Mr. Murchison and others, the Silurian system, 

 which, though composed in part of limestones, em- 

 braces also some varieties of shale and sandstone. 

 Of these, 14 species are plants, chiefly ferns, sea- 

 weed, and rushes ; 87 Polyparia ; 34 Crinoidea ; 206 

 ConchifercB ; 64 Gasteropoda; 79 Cephalopoda; 4 An- 

 nulosa; 65 Crustacea; and some fragments of fishes. 



The transition limestone occupies a narrow belt 

 of very great length in the United States ; extend- 

 ing through North Carolina, Virginia, Maryland, 

 Pennsylvania, and New-Jerse)*, it enters the State 

 of New-York in Orange county, thence passes the 

 Hudson at Newburgh, and so up on the east side of 

 the river to Hudson, where it again crosses, or, ra- 

 ther, occupies both banks, and, stretching north, 

 forms the Indian ridge elevation of several hundred 

 feet near Kingston ; thence, passing west through 

 Albany and Schoharie counties, extends northwest 

 across the St. Lawrence into Canada.* 



3. Gray wacke. 



Gray wacke is a coarse slate, containing fragments 

 of other rocks or minerals, varying in size from 

 one inch or more to the smallest grains. When the 



Professor Renwick. 



