238 CARBONIFEROUS LIMESTONE. 
This limestone is usually compact, occasionally 
granular, and containing crystals of carbonate and 
sulphate of lime, as at Niagara Falls. It sometimes 
passes into magnesian and argillaceous carbonates of 
lime. Its colour, like that of the transition, passes 
through all shades from white to black, being col- 
oured and variegated by metallic oxides, thus yield- 
ing many of the most valuable marbles. When it 
contains alumine, as it often does, it forms an ex- 
cellent hydraulic cement. It contains numerous 
fossil organic remains ; the petrifactions consisting 
of white carbonate of lime, thus causing a most 
beautiful appearance when the mass is polished. 
This limestone is very rich in minerals, such as 
lead, iron, copper, zinc, antimony, bitumen, and crys- 
tals of various earthy minerals. It is this rock which 
chiefly forms that vast formation which extends over 
the whole of the United States west of the Alle- 
ghany Mountains, and north to Lake Champlain, 
forming the bed of Lake Erie and the precipice of 
Niagara, as already described. 
Mr. Featherstonhaugh states that he ^' has traced 
the eastern border of the carboniferous limestone, 
conforming to the course of the other mineral for- 
mations east of the Mississippi, more than 1000 miles, 
running to the west of south from the State of New- 
York, to the 35th degree of north latitude in the State 
of Alabama ; the course is then changed, and lies to 
the north of west, leaving Little Rock, on the Ar- 
kansas, about 30 miles to the south, and disappearing 
between 500 and 600 miles from the Rocky Mount- 
ains. This deposite extends uninterruptedly a geo- 
graphical distance of at least 1500 miles from east 
to west, underlying portions of the States of New- 
York, Pennsylvania, Ohio, Indiana, Illinois, Missou- 
ri, and the Territory of Arkansas on that line. In 
Tennessee, Kentucky, Virginia, and Maryland, it is 
bounded by a line of which the Cumberland Mount- 
ains form a part. In the plains through which the 
