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 
sto have been formed by the agency of water perco- 
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-Jersey, 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. Graymacke, 
Graywacke 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. 
