OF THE EARTH AS A MASS. 
17 
4. Mica, or as it is commonly called, Isinglass, is distinguish¬ 
ed by being easily separable into thin, glittering, elastic plates or 
laminae. 
5. Hornblende.— \\. is to the presence of this mineral in a 
state of minute division, that the black rocks, called by the plan¬ 
ters and farmers, iron rocks, owe their color. It also occurs un¬ 
der the form of small irregular crystals, disseminated through 
other rocks, and when they are decomposed, it appears as a black 
sand, such as may often be seen in the roads after a rain. On 
the banks of Salmon CrecU, in Bertie county, isfound ablack sand, 
consisting of particles of hornblende, with hardly any admixture 
of a diflerent substance. This is the sand used to absorb the ink, 
and prevent blotting when haste is required in the transaction of 
business. In Ashe it forms whole rocks. This mineral abounds 
in the black oxide of iron. 
6. 7. S. Talc, Steatite and Chlorite, from their intimate re¬ 
semblance, may be regarded as varieties of the same mineral.— 
They all contain a considerable quantity of magnesia, to the pre¬ 
sence of which they appear to owe their distinguishing characters. 
They are all so soft as to be easily impressed by the nail, and 
their powder has an unctuous feel. Talq is in thin transparent 
scales like mica, but is not elastic, and has a pearly lustre. Chlo¬ 
rite is so called from the Greek because the scales of 
which it is composed, are always of a green color. Steatite is 
common soapstone. 
9. Serpentine, is in most cases easily distinguished by its 
structure, which is massive, its color, which is generally some 
shade of green or yellow, and its hardness, which is such as to 
admit of its being cut with a knife. If is found in the northern 
part of Wake, and in tlie counties beyond the Elue-Hidge in 
several places. 
10. Carbonate of A/me.—Common Limestone needs no par¬ 
ticular description. It is distinguished by the eti’ervescence pro¬ 
duced when a strong acid is poured upon it. 
11. Sulphate of Lime, or Gypsum, is a recent rock, and has 
never yet been discovered in North Carolina. 
l i. Iron is the coloring matter of most rocks and soils. 
13. Water. —About three-fourths of the surfiice of the globe 
are covered with water. The mean depth of the sea has been 
estimated at two or three miles, but nothing is known upon the 
subject. 
These minerals compose the outer crust of the globe, and have 
been described under the head of “ Composition of the Earth f 
because there is no evidence that its interior do not resemble (ex¬ 
cept so far as their condition is affected by their temperature) its 
exterior parts. But it is the exterior crust of the globe that is 
the principal object of the science of Geology, and this affords an 
ample and interesting field for investigation and inquiry. 
