The ONYX MARBLES. II

Their Origin, Composition, and Uses, Both Ancient and Modern

By George P. Merrill

Curator, Department of Geology, U. S. National Museum.

In discussing the origin of the onyx marbles, it is perhaps but fair to question the possibility of their having been originally deposited as tufaceous materials, and subsequently compacted and crystallized by pressure, heat and percolating solutions, or other of the ordinary agents of metamorphism. This can be best answered by pointing out the alternating character of tufaceous and compact layers. It is difficult to conceive of conditions such as should have metamorphosed any one bed without affecting, in the least, those either above or below, or both. The wavy, undulating lines of deposition, comparable with the rings of growth upon a tree, are well defined and unbroken, and though differing frequently from one another in color and crystallization preserve their parallelism and individuality throughout.There are apparently good reasons for supposing the material to have been deposited as we now find it, the crystallization being contemporaneous with deposition, as is the case with the stalagmitic material in caves. The wonderful variations in color, even in the same block or slab, are, however, in part due to changes subsequent to the deposition, and it may be well to dwell upon this branch of the subject in considerable detail. Few rocks possess so wide a range of colors or shades of the same color. Pure white, opaque, milk or chalk white to almost colorless, gray, brown in hues from light ochre to deep mahogany, buff, amber, ochre yellow, pink, red, and green are all common; the various hues being sometimes constant throughout large masses, sometimes intermixed and blended, sometimes occurring in alternating parallel bands, and sometimes in distinct veins and spasmodic dashes. In the majority of cases the coloring matter is supposedly iron in some of its forms, aided in part by manganese; in any case the apparent color may be modified by conditions of crystallization and structure, a clear translucent stone, by transmitted light, appearing much lighter and of more delicate tints than one that is opaque. Assuming that the color constituents are only the two metals named, it yet remains to account for their presence and explain the conditions under which they give rise to such a variety of hues. It is easy and presumably correct to assume that the coloring matters were deposited contemporaneously with the calcite, but we must not in accepting this overlook the fact that these substances are unequally soluble, and under proper conditions would not be deposited together at all, but would undergo a process of natural separation. As is stated by Bischof, water which contains carbonate of lime and protoxide of iron "may, when it passes for long distances in contact with the air, finally deposit pure carbonate of lime." This, for the reason that the iron early becomes converted into the condition of sesquioxide and is deposited almost at the start. We can here account for their intimate association only on the supposition that at the time of deposition the water was not flowing, but lying in quiet pools where oxidation as well as loss of carbonic acid was retarded. The variation in color of the bands might thus be in part accounted for on the supposition that the waters, as they issued, contained at times varying amounts of the oxides mentioned (See pl. 2).

In fact, however, the varying hues are by no means due wholly to the proportion of metallic oxides, but rather to the conditions under which these oxides exist and to organic matter. In certain cases, as in the slabs shown in plate 2, the bands are of alternating white, green and brown color, though all show practically the same percentages of iron when calculated as protoxide carbonate. As a matter of fact, however, the iron exists in this state only in the white and faintly greenish layers, the brown layers containing it in the form of both protoxide and more or less hydrated sesquoixides. Bischof, in speaking of a like condition in the so-called sprudelstein of Carlsbad, says:

The brown contains a considerably larger quantity of peroxide of iron than the white, which is sometimes quite free from it. This difference presupposes either that there is a difference in the quantity of iron contained in the water, or that sometimes the atmospheric air has a greater influence than at others, and that in those cases a larger quantity of protoxide of iron is peroxidized.

In the case of the stones here described the percentage of iron in the green and brown and red varieties is nearly the same, the varying hues depending mainly upon its chemical condition. While it is possible that a part of the change from protoxide carbonate to sesquioxide took place at the time of deposition, a large part is due to oxidation which has taken place since the beds were in substantially their present condition, and is due to percolating solutions. That this is the case is abundantly proven by the fact that the oxidation in most cases can be readily seen to have progressed along lines of jointing and fracture, and along the more porous layers. In many cases the oxidation is accompanied by a partial removal of the lime carbonate, whereby the stone is rendered cellular and unfit for use. Such, however, is not always the case, and many of the oxidized varieties are beautiful in the extreme, as well as unique.

The cave marbles differ from the travertines mainly in method of deposition, being cold-water deposits upon the walls and floors of limestone caves. Rain-water passing through the atmosphere and soaking through the layer of soil by which the earth is covered becomes charged with a varying amount of carbonic acid, which gives it the power of dissolving slowly the lime carbonate forming the essential constituent of the rock limestone, as already noted.

Filtering downward through cracks and fissures or between the laminæ composing the beds, it thus gradually enlarges them until what are popularly known as caves or caverns are produced. But after this cave-forming process has gone on for awhile another process sets in, whereby the cavern may be wholly or in part refilled. The water from the surface percolating down through the roof of the cave dissolving out a portion of the lime carbonate, just as when running through a crack or fissure, but in this case the water comes through the overlying rock and remains for a time suspended, in the form of a drop, from the ceiling. Here it evaporates or loses a part of its carbonic acid, and unable longer to hold the lime in solution, begins to deposit it in the form of a ring around the outer margin of the drop. As time goes on this ring becomes prolonged into a quill-like tube, growing in length away from its lower end. After a time, as a rule, this frail tube becomes partially or wholly closed, when the water flows down over the outside, the growth now being wholly external. In this way are formed the elongated pendant cones from the roofs of caves, and to which the name stalactite is given. Such on being cut and polished show a beautiful zonal structure, not wholly unlike the rings of growth upon the trunk of a tree. (Plate 3.)

But it rarely happens that all the water evaporates upon the ceiling; a portion usually falls upon the floor, where by a similar process it builds up a deposit chemically the same as the stalactites, but differing in that owing to the spreading out of the water as it falls, the floor deposits are more massive in form. To these floor deposits the name stalagmite is given. In some cases they rise in the form of blunt trunks or cones to meet their corresponding stalactites above until there are formed continuous pillars from floor to ceiling, as shown in plate 4. If this process goes on for a sufficient time the entire cave may be filled, and since the water in percolating through the roof dissolved only the pure lime carbonate, or with but a trace of impurity, leaving nearly all the carbonaceous, siliceous, and clayey constituents behind, so these stalactitic and stalagmitic deposits are of purer lime, refined by nature's methods and recrystallized under new conditions. The form of crystallization, it should be stated, is sometimes that of aragonite, but so far as the writer's experience goes, more commonly that of calcite. It is sometimes, though not always, possible to distinguish between the two forms of crystallization by the unaided eye, stalactites (or stalagmites) of aragonite showing interiorly a radiating fibrous structure, the fibres being not infrequently beautifully curved and of silky luster, while those of calcite are more granular. It sometimes happens that deposits of both kinds are to be found in the same cave, though so far as my own observation goes they belong in such cases, as at Wyandotte, Indiana, to different periods of growth. What the conditions are upon which these varying forms of crystallization depend is not now apparent.

It follows almost from necessity from their mode of origin, as above given, that the beds of onyx marbles, both spring and cave deposits, are as a rule far less extensive and regular in their arrangement than are the ordinary stratified and bedded limestones and marbles. Spring action is more or less intermittent, and the place of discharge, as well as the character of the deposit, variable. The latter usually take the form of a comparatively thin crust, conforming to the contours of the surfaces on which it lies. The various layers thicken and thin out irregularly and are often lenticular in cross section. Sound and homogeneous layers of more than twenty inches thickness are not common. Where two or more layers of sound and merchantable material occur they are not infrequently separated by tufaceous material, foreign debris, or by impure and porous onyx of little value. This condition of affairs will become more apparent as particular occurrences are described. The cave marbles vary even more irregularly both in extent and quality. The deposit may be a mere veneering over the face of the rock, and although there is apparently an abundance, judging from appearances alone, the actual amount of available stone may be extremely small. Moreover, such deposits are rarely uniform for any great distance, either in texture or color. Owing to coarse crystallization they fracture easily, and, moreover, are more than likely to contain numerous cavities, large and small, known as "thumb."."pin holes." The small amounts of metallic oxides and organic.they contain render the colors light and usually dull. White, yellow, amber, and reddish, with a resinous luster, are common. The rock as a rule is less translucent than the true onyx marbles, and when polished appears "muddy" and unsatisfactory. Nevertheless, such deposits do not infrequently yield comparatively small blocks of beautiful material and material that is doubly desirable because it is unique.

Properly managed such can be worked up to good advantage, but too much has been expected from them, and it is this fact that has led to the disastrous failures following every attempt that has thus far been made to work the cave marbles in America. If the material as taken from the ledge could be assorted by some competent person and worked up, each block for such a purpose of ornamentation as it seemed best adapted, then we might hope for some interesting results. But at best the cave marbles of America must rank as "uniques" rather than objects of commercial value. They will never become regular sources of supply. There is too much waste and too much uncertainty regarding amount and quality.

A marked and very beautiful feature of the onyx marbles in general.of those which originate as spring deposits, is the fine,.parallel bands of growth or lines of accretion shown on a .section, and which are of course due to its mode of origin through successive depositions upon the surface (See Plate 1). The stone owes its chief value for decorative purposes to its translucency, fine veination, and color. In many instances the original hues have become enhanced by oxidation and through the development of reticulating veins of small size, due to incipient fracture, into which percolating waters have introduced new coloring solutions or locally oxidized the protoxide carbonates, which seem to form the chief coloring constituent, as already noted.

The localities from which the finer grades of stone of this type have in times past or present been obtained are few and widely scattered, and it is interesting to note that, with the exception of the cave deposit, all that have thus far come under the writer's notice which are of such color as to make them preëminently desirable for ornamental purposes, occur in hot and arid countries and regions not far distant from recent volcanic activity. This is as true of foreign as of American occurrences. It is to be noted that all the deposits known are of slight geological antiquity, belonging to late Tertiary and early Quarternary periods. If materials of like nature were earlier deposited they would seem to have so far lost their identity as to be no longer recognizable. Contrary to the general believe, as indicated in the literature of the subject, or by the labeling of samples in museums, the onyx marbles, as shown by the investigations here chronicled, are almost without exception of calcite and not aragonite. It is true that the basis for such a statement is founded mainly upon specific gravities, the results of which may in certain cases seemingly be open to doubt. While, however, it is possible that certain of these stones may be made up of finely alternating bands of calcite and aragonite, there would seem no legitimate reason for doubting the main mass of the material to be calcite, particularly when microscopic examinations have borne out the results obtained by gravity methods.

George P. Merrill

Curator, Department of geology, U. S. National Museum.

(to be continued.)

(NOTE: The continuation of this article is not available.)