ART. XX.-Cone-in-Cone; by W. A. TARR.1 Cone-in-cone is a structural feature found in shales and rarely in coal. It is usually associated with concretions but not necessarily so. An occurrence of cone-in-cone in coal and its development in bands of calcite are such exceptions. The cone-in-cone structure consists of a series of cones within cones, adjacent cones uniting to form lenses or layers. When associated with concretions the cone-in-cone may occur on the upper or lower surface, or, more rarely, within the concretion. The structure was recognized and called cone-in-cone in the early part of the last century. It was not so called by all, however. Hildreth, in 1836 (see literature at end for all references), described it as a "fossil columnar Madrepore." It has also been called "cone-in-cone coral." The German name for cone-in-cone is "tutenmergel" and was given to it as early as 1823. Cone-in-cone has been described by Marsh, Sorby, Newberry, Jukes, Dawson, Daintree, Young, Sach, Garwood, Gresley, Broadhead, and many others. Their descriptions are all very similar, although their conclusions regarding the origin of cone-in-cone differ considerably. Probably the majority of investigators have regarded cone-in-cone as having been caused by pressure which was due (since the structure is so frequently seen in association with concretions) to the expansion of concretions through growth. Thus, most cone-in-cone structures are regarded as of secondary origin. Some of those holding this view are Marsh, Geikie, Dana. Gresley, Grimsley, and Chadwick. Others have regarded the structure as being due to crystallization or to "imperfect crystallization." Owen, Newberry, Geikie, Sach, Grabau, and Keyes have advocated one or both of these methods. Odd suggestions have been made by Sorby, who supposed that oolites had formed the sides of the cones; by Daintree, who believed the cones were chemical precipitates; and by Young, who says unequivocally, that they 1 The writer wishes to acknowledge his indebtedness to many friends for material furnished for this study. Mr. H. L. Griley and W. H. Twenhofel have furnished some excellent material. The late G. C. Broadhead collection at the University of Missouri contained several excellent small specimens. are due to gases rising through the muds. A view similar to that of Young has been expressed by Lawson. This view has been adequately disproved by Gresley and Harker. After a careful review of the literature on cone-in-cone the writer feels that there are still many points upon which more data are desired. He has studied a suite of specimens from many places, and it is the object of this paper to give the results attained, in the hope that others will give some attention to a very difficult, yet extremely interesting problem. Description of Cone-in-Cone. The general features of cone-in-cone are essentially the same for all occurrences. This similarity is true even of the minor features of the structure. The structure con FIG. 1. A FIG. 1.-Cone within cone. FIG. 2. sists of a series of right circular cones, which may fit one inside the other (fig. 1). This is the common mode of occurrence, but cones may also occur singly (fig. 2). They are usually grouped along a plane (fig. 3), which, if it occurs on the surface of a concretion, may be curved to fit the surface (fig. 4). Cone-in-cones are usually associated with concretions and in areal extent are generally co-extensive with them. The writer has observed this structure in concretions five feet in diameter. The cones are generally perpendicular in the central part of the layer (at a in fig. 4), but near the edge they are inclined (as at b, fig. 4). The height of the cones varies from a thirty-second of an inch to eight or nine inches. Those from one to four inches in height are the most common. The diameter of the base depends upon the height and upon the angle of slope of the sides of the cones. In many cones this diameter is nearly equal to the height of the cone. The included angle (a, fig. 2) at the apex of the cone ranges from 25° to 70°. If the cone is well developed this angle is generally 60° to 70°. Partially developed cones are sharper. FIG. 3. AAA FIG. 3.-Cones along a plane. The sides of the cones are rarely perfectly smooth. Striations such as are seen on slickensided surfaces are common. What Gresley calls "conic scales" (fig. 5) are also very common. The sides of the cone may be ribbed or fluted, thus giving a notched outline on the base (fig. 6). FIG. 4. FIG. 4.-Concretion with layer of cone-in-cone (scale 1 inch equals 1 foot). The inside of a cone into which another cone fits is always ribbed with circular rings, which are darker in color than the material composing the mass of the conein-cone (fig. 7). These rings are always on the inside of the cone. They vary in width from mere lines to over one quarter of an inch. Within a given cone they are very fine near the apex, and coarsest near the lower edge. This holds true for all specimens examined by the writer and has been so described by others. These rings are composed of clay, usually dark as mentioned above, and are free from carbonates. The ring of clay occupies a depression on the inside of the cone (fig. 8, A and B). This clay is similar to the insoluble residue of the conein-cone. The rings are broadest and most numerous in the more impure specimens of cone-in-cone. The ridges between the rings are striated and have the same slope as the outer surface of the cone, fitting into the cone-cup. FIG. 5. FIG. 6. conic scales conic scale FIG. 5.-Conic scales on the side of cone. FIG. 6.-Basal outline of a cone, with conic scales on the sides. The cones may be oblique especially near the edge of a cone layer. Some may have the apex removed and others may be more or less bent and twisted. A peculiar feature of the cones in layers, not associated with concretions, is the flaring of the base and the acute apex (see fig. 9). Such cones are common in Bond Co., Illinois, and in some thin lenses of cone-in-cone in Boone Co., Missouri. The fibers composing the cone-in-cone layers are parallel or inclined. If the latter, they are parallel to the surface of the cones. FIG. 8. B FIG. 8.-Section of clay rings showing relationship to fibers of calcite. A-natural size. B-enlarged three times. The composition of cone-in-cone is significant. Analyses show from 60 to 98 per cent CaCO3, with the remainder usually clay and other insoluble materials. The common occurrence of cone-in-cone in shales and FIG. 9. FIG. 9.-Cone showing flaring of base. always in association with calcareous portions of the shales would account for the presence of the clay. Probably the association of cone-in-cone with calcareous concretions has a bearing on their occurrence. The writer has found them in thin lenses of fibrous calcite in the Pennsylvanian shales in Boone Co., Missouri. |