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 talc. 



2. Dolomyte. — Not distinguishable 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 T. , that of Canaan, Con- 

 necticut, and of Lee and Stockbridge, Massachusetts. 



n. Crystalline Rocks, exclusive of Limestones. 



The crystalline rocks may be distributed according to 

 their composition into the following series or groups. Each, 

 excepting the first, embraces both metamorphic 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 disposition — tripolite or diatom beds being made 

 chiefly of opal-silica which is readily soluble in waters 

 slightly alkaline. 



2. The Mica and Potash-Feldspar series. These are emi- 

 nently alkali-yielding rocks, both the mica, whether mus- 

 eovite, 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, 

 belong with this series. 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). 



