Birds and All Nature, Vol. 6, No. 2, September 1899 / Illustrated by Color Photography

by Various

	MARBLES.	CHICAGO: A. W. MUMFORD, PUBLISHER.
OLD TENNESSEE.		ALPS GREEN.
SIENNA.		MEXICAN ONYX.
FLORENTINE VERMONT.		AFRICAN MARBLE.

MARBLES

MR. GEO. D. MERRILL,
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.