434 DESCRIPTIONS OF ROCKS. 
the 500 quarries only 20 furnish stone for the sculptor. The amount 
of marble taken out from the quarries in 1876, was 120,000 tons, 
valued at $2,400,000; and of this 40,000 tons came to the United 
States. The Cipolin marbles of Italy are white, or nearly so, with 
shadings or zones of green tale. 

able by the eye from gran- 
ular limestone. 
Part of the marbles above referred to are dolomyte. This is the 
case with that of Westchester County, N Y., that of Canaan, Con- 
necticut, and of Lee and Stockbridge, Massachuseits. 
II. Crystalline Rocks, exclusive of Limestones. 
The crystal:ine rocks may be distributed according to 
their composition into the following series or groups. Each, 
excepting the first, embraces both metamor phic and eruptive 
rocks. 
1. Siliceous rocks, The kinds consisting mainly of quartz 
or opal are here included. The first of “those mentioned, 
page 435, is intermediate between the fragmental and meta- 
morphic-crystalline rocks. The opal material is a chemical 
deposit. ‘The chert of sedimentary formations is believed 
to be mainly tripolite consolidated through the solution of a 
part of its material by the permeating waters and its sub- 
sequent desposition—tripolte or diatom beds being made 
chiefly of opal-silica which is readily soluble in waters 
shghtly alkaline. | 
5. The Mica and Potash-Feldspar series. These are emi- 
nently alkali-yielding rocks, both the mica, whether mus- 
covite, biotite, or lepidomelane, and the feldspar, whether 
orthoclase or microcline, affording on analysis, as explained 
on page 411, much potash, and the feldspars often also some 
soda. The soda feldspar, albite or oligoclase, is a common 
accessory ingredient. The series shades off into a rock 
that is chiefly feldspar, and another that is chiefly mica ; 
and in these two extremes the amount of potash yielded is 
about the same. Moreover, as leucite is essentially a potash- 
feldspar in ratio and composition (see page 411), rocks, con- - 
sisting chiefly of leucite, without pyroxene or hornblende, 
Lelong with this serics. Muscovite and biotite commonly. 
occur “together, the formation of biotite having been deter- 
mined by the presence of some iron oxide in “the original 
material from which it was made. The mica is sometimes 
a hydrous species (page 313). 
