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[Assembly 
at the bottom of wells, cisterns or caves. Its presence may always 
be known by letting down a candle, by which it will be extinguished; 
a precaution which should always be observed previous to descending 
into them. A person is rendered entirely incapable of exerting him- 
self the moment he inhales this substance, and of course cannot extri- 
cate himself when immersed in it. 
Carboniferous, Coal-bearing. 
Chloride. A combination of chlorine and a base, as sodium, which, in 
chemical language, is called a chloride of sodium ; in common lan- 
guage, it is dry sea salt. 
Chert. A silicious mineral resembling flint, hornstone, &c. Rocks con- 
taining it, are called cherty. 
Chlorite. A soft greenish mineral composed of minute scales closely 
compacted together. It differs very little from soapstone. Rocks 
containing it, are said to be chloritic. 
Clinkstone. A rock alhed to basalt, which rings when struck. 
Cleavage^ is the separation of the lamina of rocks or simple minerals in 
certain constant directions. In rocks, it is not always parallel to the 
planes of stratification; and hence the planes of cleavage may be mis- 
taken for those of stratification, and vice versa. 
Coal measures^ coal formation. Rocks containing coal. 
Conformable. The arrangement of the strata of two rocks in parallel 
position. 
Conglomerate. Masses of rounded stone, gravel and dirt, cemented to- 
gether. 
Cretaceous, Belonging to the chalk formation. 
Crop out, and out crop. Emergence of a stratum or a rock from the soil, 
or from the adjacent strata or rocks; in other words, its appearance at 
the surface. 
Crystal^ crystalline. A geometrical solid; the latter, an assemblage of 
imperfectly defined crystals. 
Crustacea. Animals having a thin coating or crust, like the lobster, which 
they shed periodically. 
Delta, Alluvial lands formed near the mouths of rivers, of a triangular 
shape, and included between the branches of the river and the sea- 
shore. Large rivers, just before their entrance into the sea, frequently 
divide into several branches; this division is in consequence of the 
earthy matter borne down by the river to its junction with tide water, 
where it is deposited, forming thereby banks of sediment which rise 
in time above the water. 
Denudation. A term used to express the removal of sedimentary mat- 
ter from any area. Rocks from which any covering: has been swept 
off by currents of water, are said to be denuded. Valleys are some- 
times formed in this way; they are then called valleys of denudation. 
Deoxidized, Deprived of oxygen. 
Dicotyledonous, One of the grand divisions of the vegetable kingdom, 
founded on the form and structure of the seed, which, as the name 
expresses, has two seed lobes. 
Diluvium. An accumulation of loose sand and gravel, and other mate- 
rials, which some geologists suppose to have been collected or spread 
Qver a district by the action of a deluge. 
