102 GEOGNOSY AND GEOLOGY. 
There is, perhaps, no other to be compared with it in this respect. It not 
only forms immense masses in particular localities, but is universally 
distributed, this being the case both with respect to the pure varieties and 
to the mixed. 
Section 1. Limestone Proper. 
This section includes all rocks composed of pure carbonate of lime. The 
principal modifications are : 
Marble, oy pure carbonate of lime in a crystalline granular state of 
aggregation. The name of marble is often applied, vulgarly, to rocks which 
are not entitled to it. It is not upon the markings of a limestone, whether 
close-grained or crystalline, that the title of marble, geologically speaking, 
is based, but simply upon the condition of aggregation. A marble may, and 
indeed often does, have various kinds of coloration or other markings, but 
then it is not every limestone thus marked that is a marble. The markings 
often depend upon the penetration of the limestone by other matters. 
Common marble is pretty generally distributed in various degrees of fineness 
and purity. The hardness varies considerably, one extreme being as 
conspicuous as the other. The use of marble in building and sculpture is 
well known. The white variety, as it occurs near Carrara, in the Appenines, 
in the island of Paros, and in Mount Pentelicus in Attica, is the most 
esteemed. This is not so frequent as the yellow, greenish, grey, and 
bluish modifications. It sometimes appears black, and then passes into 
anthraconite. The presence of various colors imparts a spotted, pitted, or 
veined appearance. Marble contains various incidental ingredients which 
sometimes cause it to deteriorate in value. Iron pyrites is sometimes so 
intimately combined as to escape detection. Under the influence of the 
atmosphere a hydrated oxyde of iron is formed, which imparts a brown color 
to the surface. Hence it is that many columns or structures of marble 
become coated with a yellowish tinge. Augite, schorl, feldspar, hornblende, 
mica, &c., are also frequently met with, and have given rise to the 
separation of various rock species, which, on account of their limited 
occurrence, are not generally recognised. This is the case, for instance, 
with the feldspathic, pyroxenic, pyropian, amphibolic, and cipolin caleciphyre 
of A. Brogniart. 
Breccia Marble is a combination of angular fragments of marble by a 
calcareous cement. The two portions are generally of a different color. 
A weathering of marble is out of the question, as the atmosphere is 
incapable of causing the decomposition of carbonate of lime. A crumbling 
away of the rock may, however, be effected by the decomposition of the 
interspersed sulphuret of iron, and of the carbonates of metals, causing a 
necessary swelling of particular portions at the expense of others. 
Compact Limestone. This form of calcareous matter is the most 
important, in a geognostical point of view, and embraces a large number of 
varieties. 
The common compact limestone possesses, on a large scale, a conchoidal, 
on a small, a splintery fracture. It is mixed more or less with other 
substances, especially with particles of clay, which cause a diminution of 
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