MINERALOGY. 77 
(figs. 75 and 76, or pl. 35, fig. 26). Complete crystals possess not 
unfrequently a twin structure, as shown in fig. 2, pl. 35.  Feldspar is 
readily cleft in two directions, the surfaces thus exposed being lustrous. 
Its fracture is conchoidal, uneven; color white, to flesh color. The white, 
yellow, flesh colored, or ferruginous portions of granite, are feldspar, which, 
with quartz and mica, compose this extensively distributed and important 
rock. The feldspar of granite decomposes surely though slowly; and its 
potassa, combining with water and silica, becomes a principal source to 
plants of this essential inorganic constituent. The rest of the feldspar, 
richer in alumina, and poorer in silicic acid and potassa, is also an important 
item in the soil; alumina being essential in rendering a sandy soil fruitful. 
Kaolin is also decomposed feldspar, and is found abundantly in granite. 
Feldspar is closely allied to the preceding mineral, namely, albite, when a 
portion of its potassa is replaced by soda, as in pericline. Closely allied to 
feldspar are oligoclase and spodumene, the latter containing lithia in place 
of potassa. Pl. 35, fig. 1, represents a specimen in granite from the island 
Uto. Labradorite, in which lime and soda replace potassa, belongs also to 
the feldspars. 
Class 10. Salts. 
The minerals beionging to this class are non-silicious combinations of an 
acid with oxydes; from which an acid may be derived by boiling with 
caustic potassa, or by melting with carbonate of soda; or which exhibit an 
effervescence of carbonic acid gas when exposed to the action of certain 
acids. 
1. Carbonate of Lime. 
Carbonate of lime is one of the most important of all minerals: it is a 
constituent of many rock-formations; is dissolved in all waters in the form 
of a bicarbonate; and as such, in combination with other acids found in 
the soil, enters as a necessary ingredient into the organism of plants and 
animals. Water containing carbonate of lime in solution is capable of 
dissolving fresh carbonate of lime, especially under pressure ; such as would 
be produced on the lower end of a spring of water, in coming from a 
considerable depth below the surface. The pressure removed, and the 
water exposed to the concentrating influence of atmospheric evaporation, 
the necessary excess of lime will be deposited, in a crust, on all bodies wet 
by the fluid. Where, as in caverns, such water drops continually from a 
certain point, a deposit of carbonate of lime will be left behind, and will 
accumulate in time so as to form a considerable mass, resembling an icicle 
in shape. Such formations are called stalactites; and the less regular 
deposit of the same carbonate, which usually forms beneath the stalactite, 
is called stalagmite. Fig. 13, pl. 35, represents the initiatory stage in the 
formation of such stalactites, where the rock has become covered with a 
coating of stalactital matter, with depending projections. Fig. 28 
represents a stalactite after it has attained its characteristic form. 
Carbonate of lime is the actual constituent of limestone, chalk, and 
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