partially penetrated, by small columns (see Figs. 28 and 29). Polished surfaces of the Tennessee marble often offer splendid examples of this (see Fig. 30). Many specimens of fossils which have been penetrated by stylolites are found in the Columbus limestone of Ohio. One of the most striking ex

FIG. 30.-Large shells (shaded portions) penetrated by stylolites. Specimen is from a polished slab of Tennessee marble in the Monroe County State Bank, Bloomington, Ind. Three-fourths natural size.

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FIG. 31.-Diagram of a stromatoporoid into which a series of stylolites have penetrated. Note that the fossil structure has been actually removed where the upward-pointing columns occur. From a specimen in the Geological Museum, Ohio State University. About one-third natural size.

amples, observed by the writer, is that of a stromatoporoid into which a series of columns of considerable size have penetrated (see Fig. 31). Here is a conclusive case of part of the fossil structure having been actually removed, and occupied by the upward-pointing columns of the rock below. The residual clay is found in its place at the end of the columns.

These numerous, partially cut fossil shells, mineral grains, etc., mentioned above, show no evidence of compression or disturbance from their original positions. They have been cut after the rock material was firmly hardened and cemented together. These observations alone preclude all other theories of the origin of stylolites.

PENETRATION OF ONE STYLOLITE-SEAM BY ANOTHER ADJACENT PARALLEL SEAM. The vertical distance between stylolite-seams is quite variable. It is sometimes as small as a few inches. Sometimes the seams are so close as to touch, penetrate, or pierce one another. In all cases, parallel stylolite-seams, as they grow, become closer together by a distance dependent upon the amount of solution which takes place. Thus, two parallel seams, which in their beginning were separated by a very thin layer, might, after sufficient solution and growth, become so close as to touch one another (see Fig. 32). Continued solution would result in the interpenetration of the two, and still further growth would cause a partial, or complete, eradication of one or the other. These various stages are observed in the Indiana limestones. The above-mentioned Fig. 25 is an example where a large, major stylolite-seam has partially destroyed a small, minor one. Individual stylolitecolumns of various sizes, which bear a partially penetrated column of an overlying or underlying seam, have been observed. Wagner (1913, p. 118) cites the example of an older curved stylolite which has been pierced by a younger vertical one (see Fig. 33). All this evidence is decisive proof of the actual removal of rock material, and presents a phase of the problem which can be explained by no other theory than solution.

The

STRIATED AND SLICKENSIDED FACES OF STYLOLITES. ever-present striations on the side-surfaces of stylolites, running parallel with the direction of penetration, present a problem hard to explain by the pressure, and other theories.

FIG. 32. Two parallel stylolite-seams of the Salem limestone, which, in places, touch and partially penetrate one another.

Still more difficult to account for by the pressure theory are the numerous polished and slickensided mineral depositsusually calcite-on the sides of the columns (see pp. 50 and 88). There can be no question but that this mineral matter was deposited there after the rock had become hardened. Further movement of the columns past one another then resulted in the polishing of such deposits. That striations and slickensides were developed while the rock was yet in a soft plastic state appears to be a physical impossibility. They result from the slow slipping of the face of one column along that of the adjacent one, in hardened rock, such as takes place along a fault surface. Striations of stylolites of coarsegrained stone are deeper and coarser than those of finergrained rock.

FIG. 33. Example of an older, curved stylolite pierced by a younger, vertical one. (After Wagner.)

If stylolites were formed in soft, plastic sediment, as explained by the pressure theory, should not one expect the sides of the alternating columns to be intercemented at the time of the hardening and cementation of the entire rock mass? Such is not the case.

DIRECTION OF STYLOLITES AND STYLOLITE-SEAMS. The pressure theory would require that stylolite-seams be developed along bedding or lamination planes where a film of clay has been deposited. Differential compression of the plastic, or semi-plastic, mass would thus produce vertical columns at right angles to the bedding planes. However, stylolite-seams are developed along inclined bedding planes with the columns not at right angles to them (see p. 54). Such stylolites have been formed subsequent to the folding and tilting of the strata. Folding obviously either occurred after

the rock had become consolidated, or was responsible for the consolidation. That the pressure theory fails to explain such an occurrence of stylolites is evident.

The pressure theory also fails to explain the origin of stylolite-seams which cut across the stratification at various angles (see Fig. 17). If adherents of the pressure theory would have the seam developed along an angular crevice, it would fall upon them to explain the origin and existence of such a crack in plastic rock. Equally difficult would it be to explain the occurrence of a clay layer along such a crevice (see p. 86).

2. Evidence that the Clay Partings Are the Solution Residue of the Dissolved Limestone

The

The general assumption of most investigators of stylolitic phenomena, with the exception of the exponents of the solution theory, is that the ever-present clay partings of styloliteseams represent original, once-continuous, thin layers of clay material laid down in the seas with the lime sediments. solution theory holds that the clays are the residue of the dissolved rock, altho advocates of the theory, however, have never given conclusive proof of this. The writer wishes to present several lines of evidence showing that the clays are a solution residue.

CHEMICAL RELATIONS BETWEEN THE CLAY CAPS AND THE ASSOCIATED LIMESTONE. No investigator has attempted an analysis of the ever-present clays of stylolites to show the chemical relations between them and the associated limestone (or dolomite or marble). If the clay caps are the solution residue of the dissolved limestone, one should expect a definite relationship between their chemical constituents and those of the limestone from which they were derived. The clay should consist, in the main, of the least soluble components of the parent limestone, with probably a subordinate amount of the soluble substances which have not yet been completely dissolved; since, in the solution of limestone, only the calcium carbonate is removed in appreciable quantities. The clay, if a solution residue, should be nothing more than a concentration of the less soluble constituents of the rock from which it was derived. It should contain these substances in a proportion dependent primarily upon the amount dissolved and carried away during the weathering of the parent rock.