Head Curator, Department of Geology, U. S. National Museum.
The origin of the name marble, like that of many another name now
in common use, is somewhat obscure. By many authorities the word is
supposed to have been somehow connected with the Greek word meaning
"sparkle." However this may be, a sparkling appearance is by no means
universal among marbles, but is limited to those which, like the white
statuary or other crystalline varieties, have a granular structure, the
sparkling itself being due to the reflection of light from the smooth
surfaces of the constituent minerals. As used to-day, the word marble
is made to include any lime rock of such color and hardness as to make
it desirable for ornamental, or even the higher grades of building
work. Stones of precisely the same composition and origin, which are
not of the desired color, are classed simply as limestones.
Accepting the definition given above, it follows, then, that with a few
exceptions, to be noted later, marbles are but hardened and otherwise
changed beds of marine sands and muds, containing, it may be, still
recognizable fragments of the corals and mollusks of which they were
originally composed. But inasmuch as these muds were rarely of pure
carbonate of lime, but were contaminated with matter from seaweeds
and animal remains, or by iron compounds, so the resultant marble is
not always white, but, if containing matter from plants or animals,
gray, blue gray, or even black; and if containing iron, buff, pink, or
red. If the change in form of the original muds was just sufficient to
produce crystallization, we may have a marble full of fossil remains
which may be of a white or pink color, standing out in fine contrast
with the darker ground. If, on the other hand, the change was complete,
we may have a marble of small granules, pure white in color, and of a
texture like loaf sugar, such as to render it suitable for statuary
purposes.
At one early period of the geological history of the North American
continent, all that portion now occupied by the Appalachian mountain
system was sea bottom, and on it was being deposited not merely
sediments washed down from the land, but, in favorable localities,
deposits of lime, sand, and mud. This deposit went on, on a gradually
sinking floor, for long ages, until the lowermost beds were buried
under thousands of feet of the later formed materials. Then began the
slow uplifting of the sea-bottom in the form of long, parallel folds
to form the mountain ranges. During this uplifting the lime sediments,
which are the only ones we need consider here, were changed to marbles,
and have since been exposed and made available to the quarriers through
the wearing-down action of rain and running streams. So, then, a quarry
is but an excavation in the hardened mud formed on the bottom of a very
ancient sea.
In the Vermont marble region the beds are highly inclined and of
varying colors. From the same quarry there may be produced pure white,
gray, blue-gray, and greenish varieties, often variously veined and
blotched owing to the collection of their different impurities along
certain lines. Some of these quarries have been worked a depth of two
hundred feet and more.
Not all marble beds are upturned at this steep angle, however, nor
have they been worked so deeply. In Georgia, the quarries are often in
hillsides, extending scarcely at all, if any, below the surface of the
ground. Where opened in the valley bottoms they have the form of huge
rectangular pits, with perpendicular walls. In Tennessee, many of the
sediments were so slightly changed that the fossil remains are still
easily recognized, and the stone is of a pink or chocolate red color,
owing to the abundance of iron.
The marbles are quarried mainly by channeling machines, which cut out
the stone in blocks of any desired size, or at least in sizes such
as the nature of the beds will allow. Blasting is never resorted to in
a properly managed quarry, since the shock of the explosion is likely
to develop flaws in so tender a material. When freed from the quarry
bed and brought to the surface the stone is sawn into the desired
shapes by means of "reciprocating" blades of soft iron, the cutting
material being sand, washed under the blades by small jets of water.
The use to which any particular marble is put is governed largely by
its price and color, though texture or grain often are taken into
consideration. The coarsely crystalline white and white clouded marbles
of southern New York, Maryland, and Georgia, are used almost wholly for
building purposes; the pink and variegated marbles of Tennessee for
interiors and for furniture; while the white and blue-grays of Vermont
find a large market for interiors, cemetery work, tiling, and, to a
much smaller extent, for building.
It was stated before that not all our marbles were changed
(metamorphosed) marine sediments. The exceptions are (1) the onyx
marbles, which, though composed of carbonate of lime, like the last,
are deposited from solution, and (2) the so-called verdantique marbles,
which are mainly altered eruptive rocks. These last differ widely from
those we have been describing, being of a prevailing green color,
though often variegated with white or red. They are, in fact, not to
be classed with the lime rocks at all. The names _verdantique_, _verte
antique_, and _verde antique_ are but varying forms of the same words,
indicating a green antique marble. The term antique has been applied
simply because stones of this type were used by the ancients, and
particularly by the Romans.
The so-called onyx marbles are, as noted above, spring deposits,
differing from ordinary lime deposits only in color and degree of
compactness. The name has also been made to include the stalagmites and
stalactites in caves, such as were used by the ancient Egyptians in
the construction of alabastrons, amphoræ, funeral urns, and various
household utensils. The material is translucent and often beautifully
clouded and veined in amber, green, yellow, and red colors. Owing to
its mode of origin it shows a beautiful wavy banding, or grain, like
the lines of growth in the trunk of a tree when cut across the bedding.
This fact, together with its translucency, has been the cause of the
wrong use for it of the name onyx, which properly belongs to a banded
variety of agate. Equally wrong and misleading is the name "oriental
alabaster," which is commonly applied to the Egyptian variety, the true
alabaster being a variety of gypsum.
The larger part of our onyx marbles comes to-day from Mexico, though
there are equally good materials of this type in Arizona and California.
The foreign supplies come in part from Egypt. Their use is almost
wholly for interior decoration, as wainscotings, and the like, and
for tops to small stands, bases for lamps, and so forth. These are by
far the most expensive of all the stones to which the name marble is
properly applied.
Some of the most noted of our foreign marbles are those of Carrara,
Italy, which are ancient sediments thought to have been changed at the
time of the uplifting which formed the Apennines. They are of white and
blue-gray colors, sometimes beautifully veined. A beautiful, mellow
yellow to drab variegated variety, very close in texture and almost
waxy in appearance, is found near Siena, and is known as Siena marble.
It is a great favorite for interior decorative work, as may be seen
to advantage in the vestibule of the new public library building in
Boston, and the rotunda of the National Library building at Washington.
Other marbles, which at the present time are great favorites with the
architects, are the so-called Numidian marbles, from Algeria. These are
of yellow, pink, and red color, and often beautifully mottled. Their
textures are so close that they take a surface and polish almost like
enamel. Since their first hardening these beds have been shattered like
so much glass into countless angular fragments, and then the whole
mass, with scarcely any disturbance, once more cemented into firm rock.
The result is such that when large blocks are sawn into slabs, and
the slabs then polished and spread out, the same series of veins, of
angular blocks and streaks of color, may be traced from slab to slab,
even repeating themselves with only slight changes throughout the
entire series.--_Nature and Art._