MARBLE. This title embraces such of the primitive, transition, and purer compact limestones of secondary forma tion, as may be quarried in solid blocks without fissures, and are susceptible of a fine polished surface. The finer the White, or more beautifully variegated the colors of the stone, the more valuable, ceteris paribus, is the marble. Its general characters are the following : Marble effervesces with acids; affords quicklime by calcination ; has a conchoi dal scaly fracture ; is translucent only on the very edges ; is easily scratched bythe knife ; has a spec.gray. of 2.7 ; admits of being sawn into slabs, and receives a brilliant polish. These qualities occur united in only three principal varieties of limestone ; 1, in the saccharoid limestone,. so called from its fine granular texture re sembling that of loaf sugar, and which constitutes modern statuary marble, like that of Carrara ; 2, in the foliated lime stone, consisting of a multitude of small facets formed of little plates applied to one another in every possible direction, constituting the antique statuary marble, like that of Paros ; 3, in many of the transition and carboniferons, or encriM itie limestones, subordinate to the coal for mation.
The saccharoid and lamellar, or statuary marbles, belong entirely to primitive and transition districts. The greater part of the close-grained colored marbles belong also to the same geological localities ; and become so rare in the secondary limestone formations, that immense tracts of these occur without a single bed sufficiently en tire and compact to constitute a workable marble.
Marbles abound in the United States both fine-grained and coarse. The quar ries in the neighborhood of Philadelphia afford a clouded handsome at Thomastown, in Maine, a similar variety occurs. Of black marble, resembling the Irish lucullite, there is an extensive de posit at Shoreham, Vermont, which fur nishes the chief supply to the States.
The bed lies directly on the shores of L Champlain, so that the blocks when lifted can be transported by water. Most of it goes to Middlebury to be polished. In the neighborhood of this last town a clouded granular marble is found : a dove colored variety is quarried at Pittsford, Vt., as also at Great Barrington and Sheffield. The white marble of Conn. and Westchester, N. Y., are too coarse and granular for building purposes, be sides having tremolite and other crystals scattered through them. Verd antique exists in Vermont and Conn. : that from the latter State being most beautiful. Variegated and shell marbles exist in the Western States ; and a beautiful conglo merate (pudding stone) or breccia is found at the base of the Blue Ridge, Md., on the bank of the Potomac, 60 miles above Washington. The inner columns of the Capitol are made of it.
Of cutting avidpolishing marble.—The i marble saw a plate of soft iron, continually supplied, during its sawing motion, with water and the sharpest sand. The sawing of moderate pieces is per formed by hand, but that of large slabs is most economically done by a proper mill.
The first substance used in the polish ing process is the sharpest sand, which must be worked with till the surface be comes perfectly flat. Then a second, and even a third sand of increasing fineness is to be applied. The next substance is emery of progressive degrees of fineness, after which tripoli is employed; and the last polish is given with tin-putty. The body with which the sand is rubbed upon the marble, is usually a plate of iron ; but for the subsequent process, a plate of lead is used with fine sand and emery. The polishing rubbers are coarse linen cloths, or bagging, wedged tight into an iron planing tool. In every step of the opera tion, a constant trickling supply of water is required.