precipitation of the excess mineral matter which can no longer be held in solution because of the weakening of the carbonic acid.

Solubility of Limestone

It is well established that calcium carbonate is nearly insoluble in pure water, but that it is readily attacked by carbonic acid (H,CO) and converted into calcium bicarbonate, H.Ca(CO3), which is quite soluble in water. Thus, since limestone is made up, for the most part, of calcium carbonate, it may be almost entirely removed by solution, leaving a residual

FIG. 19.-Limestone bowlder channeled by water containing carbon dioxide, illustrating the occurrence of differential solution similar to that required in the development of stylolitic phenomena. (From Cleland's Geology.)

clay composed of the less soluble, minor constituents of the rock-principally silica, alumina, oxides of iron, with small quantities of soda, potash, magnesium carbonate, and calcium carbonate which have not been completely dissolved. In the case of limestone, instead of there being a gradual transition from fully-formed residual clay into the parent rock, the passage from the clay to bed rock is sudden. The reason is that the clay is left as a residue from solution, and not from a gradual chemical breaking down and change of the minerals of the rock, as in the case of granites, etc. Thus a small thick

ness of residual limestone clay is a product of the solution of a much greater thickness of parent rock, the proportion depending upon the purity of the limestone.

The differential weathering of limestone is often quite striking. Since the ability of limestone to resist solution is quite variable, even thruout a single stratum, a solution surface often presents an undulating and irregular appearance (see Fig. 19).

EXPLANATION OF STYLOLITIC PHENOMENA UNDER THE SOLUTION THEORY

Of the above-discussed factors, the writer wishes to emphasize the following two as the most important in the explanation of stylolites:

1. The effect of pressure upon the solution of solids.

2. The differential solubility of limestones, and other car

bonate rocks.

Stylolites originate in carbonate rocks-varieties of limestones, dolomites, and marbles-along a bedding plane, lamination plane, or crevice, where the circulation of ground waters, charged with carbon dioxide, is most free. Here, then, solution begins. If the ability of the rock to resist solution is slightly variable on one side of the crevice or the other, the carbonic acid would, of course, attack the less resistant parts. If these more soluble portions are distributed first on one side of the crevice, and then on the other, a slightly undulating line would develop, with the undulations becoming more marked after further solution, the outstanding resistant parts of the one side fitting into the dissolved-out portions of the opposite. After the development of this undulating line, pressure (in most cases static pressure, resulting from the weight of the superincumbent strata) plays its rôle. Most of the weight of the overlying sediment is concentrated along the axis (the top and bottom surfaces) of each of these undulations. This results in an increased amount of solution at the points of increased pressure. The sloping sides of the undulations, which are freer from pressure than the tops and bottoms, are proportionately less attacked by the solvent. Increased solution of the weaker rock opposite the ends of the undulations results in (a) a

deepening of the interpenetrating parts, (b) a decrease in the pressure and consequent decrease in the solution of the sides of the undulations, and (c) a final development of vertical columns, with practically a complete concentration of the pressure and consequent solution at the ends. The sides of the columns, being free from pressure, usually are unattacked by solution. Continued solution at the ends results in a further deepening and lengthening of the interpenetrating colStriation of the side-surfaces results from the slow movement of the columns past one another. The non-soluble constituents of the dissolved rock come to rest as a clay residue at the end of each column, and serve as a further protection from solution of the resistant part. Increase in the length of the stylolites results in a proportional thickening of the residual clay. The length of the columns serves as a fair measure of the amount of solution which has taken place, providing the ends of the columns themselves have not been subjected to solution. On the sides of the stylolites, which are free from pressure and practically unattacked by solution, are often found deposits of mineral matter precipitated there from the supersaturated solvent resulting from the increased pressure and amount of solution opposite the ends. Such coatings of mineral matter are often slickensided as a result of further growth and interpenetration of the columns. The length of the stylolites depends upon three principal factors: (a) the length of time solution has gone on, (b) the solubility of the stone, and (c) whether or not solution has attacked the ends of the stylolites.

Thus it is seen that the principal factors in the development of stylolites are: (a) the presence of a crevice in the rock which permits a concentration of carbonated water; (b) the fact that carbonate rocks (limestones, dolomites, and marbles) exhibit a differential solubility; and (c) the physicochemical principle that an increase in pressure effects an increase in the solubility of a solid, as shown by Rieke, Sorby, Geikie, Van Hise, and others, and experimentally confirmed by Becke and Daubrée.

Wagner (1913, pp. 122-123) stressed the point, from the law of Henry, that an increase in pressure upon the solvent at the ends of the columns would permit an increase in the amount of carbon dioxide dissolved, which in turn would increase the amount of solution at these places. Reis pointed

out that slight tremblings within the rock might cause friction, thus creating heat, which in turn would increase the dissolving powers of the solvents. The writer believes this latter conclusion to be unimportant in explaining the development of such an intertoothed phenomenon as stylolites.

From the solution theory it can be seen that a vast number of variations in size, shape, distribution, and character of stylolites is to be expected, principally from (a) variations in the composition and lithologic nature of the rock, (b) the erratic distribution of varying soluble portions of the stone, (c) variations in the direction of pressure exerted upon the rock, and (d) the length of time solution has continued.

The spacing of the alternating, less resistant portions of the stone on the two sides of the crevice may occasionally be quite regular. This, however, would be an exception. In most cases the distribution would be very erratic, so that the resulting columns would be of varying widths. In the beginning, stylolite-seams are the small, finely serrated type. A little further solution might, because of differential solubility of, the rock on opposite sides, develop a slightly undulating stylolite-seam, each of these undulations bearing smaller penetrations in varying numbers. A continuation of the process upon these compound major undulations might result in the development of larger, major columns, whose ends might be marked with the smaller, original penetrations, such as are often observed (see Fig. 24), and still further continuation of solution might bring about a complete, or almost complete, eradication of these original, smaller, intertoothed parts. All sorts of gradations between the beginning, barely noticeable, undulating line, and the large, major styloliteseams are to be observed in the field.

If, in the gradual interpenetration of the stylolites, the less resistant portion on the one side, which is being dissolved out opposite the end of the column, changes in resistance so that it is as resistant as, or more resistant than, the penetrating part, solution might then take place in the rock on both sides, or change to the end of the column. Such variations in the chemical resistance to solution therefore often produce quite a diversity in the length and shape of the interpenetrating parts (see Figs. 15 and 26). It explains the occurrence of shorter stylolites between longer ones. If the rock on each side of a solution crevice were of uniform re

FIG. 20.

Undulating solution seam, containing three-eighths of an inch of black residual clay. This seam, within a short distance to both the left and right, becomes highly stylolitic. See Fig. 21. From a quarry of the Consolidated Stone Company, Dark Hollow district, Lawrence County, Ind.