The Blue Shale, wherever I have had an opportunity of observing, underlies the limestone of the mounds, and separates it from the Upper Magnesian limestone. It is composed of a thin even argillaceous slate, quite hard in its natural state, but more or less subject to decomposition into a soft clay, sometimes retaining its original blue color, but more usually stained yellow, and forming then what is called by the miners, a pipe clay. Its surface, from its tendency to decomposition, is always concealed by earth, unless exposed in ravines or by excavation. It extends to a greater or less distance around the mounds, and graduates by decomposition into the pipe clay, which overlies its undecomposed part, when thickest, and replaces it entirely on its outskirts. Thus at the Jamestown Mine, near the Sinsinawa Mound, it was found, in the engine shaft, immediately overlying the upper magnesian, unchanged, and itself overlaid by the pipe clay, while in shafts more remote from the mound, it was found entirely converted into the pipe clay. This bed is less open and pervious than the limestones, and consequently the water from the mounds issues in springs above it, marking the line of its upper surface.

The shale itself contains few, if any fossils, but at its junction with the upper magnesian, there is a very thin bed, (two to three inches thick,) composed almost entirely of very small fossils and concretions, usually firmly cemented by iron, and therefore called hard-pan by the miners, but sometimes softer and with a more calcareous cement. Usually one or more thinner layers (about one inch thick). of the same character are found interposed in the blue shale, within the first 2-4 feet above the upper magnesian. These fossiliferous and concretionary layers are important as serving to determine the formation of the pipe clay, overlying the upper magnesian, from the blue shale. In the shafts, at the Jamestown Mine, where the pipe clay immediately overlies the upper magnesian, these layers are found precisely of the same character and in the same position, as where the unaltered blue shale meets the same rock. In different places on the higher points, where the upper

magnesian is most complete, that rock is found overlaid by pipe clay, in which the same fossiliferous and concretionary layers are found, in the same position as I have already stated. This I have observed very perfectly at the Muddy Diggings, on high ground, north of Cassville, at the distance of several miles from the mound rocks; the nearest position of these being in the Highlands of Iowa, beyond the Mississippi. In other places, the pecu ́ liar fossils and concretions of these layers are observed on the surface of the upper magnesian, where the pipe clay is less obvious. This I have noticed in different places on the higher grounds in Hazel Green, six miles from the Sinsinawa Mound, and still farther from any other locality of the mound strata. These facts seem to indicate a former general extension at least of the blue shale, over the surface of the upper magnesian,

UPPER MAGNESIAN.

The Upper Magnesian* consists of a series of limestone beds, of great thickness, in which the greater part of the lead ore, raised in the mineral district, has been found, and from that circumstance, it has been sometimes called the mineral rock. But the other beds of limestone, underlying it, (the blue limestone and the lower magnesian,) have been found to be good lead-bearing rocks, and consequently this latter term can no longer be regarded as dis tinctive. The prevailing character of the rock in this series, is that of a light grey thick-bedded limestone; sometimes uniformly fine-grained and even compact, but more often partly fine-grained and compact, and partly coarser grained and more distinctly crystalline, or even with small geodic cavities. This latter struc ture occurs more particularly in connexion with mineral deposits, or in what is called the opening rock. In such instances, either the compact or the more crystalline portion may be the ground, through which the other is disseminated; the former as nodules or concretions; the latter as geodes or approaching such.

* This term, introduced by Owen in his first report, has been generally adopted in the mineral district, and for that reason I have preferred to retain it.

The rock of this series is generally more or less subject to decomposition, and the coarser grained portions most so, which often gives to it a peculiar cavernous character. This circumstance renders it less valuable for building, although occasionally fine-grained or compact beds occur of superior quality for that purpose. The quarry from which the Catholic Church at Benton has been erected is one of that character. This rock too, in the openings, is often found decomposed in part to a fine sand, retaining its structure unchanged, in which the harder compact concretions lie loose in their original position, and are called tumbling rock by the miners. It has been called, from this circumstance, sand-stone and sand-rock, by the miners, but as these names are liable to confound it with the proper silicious sandstone, they should be rejected.

There is generally a thin bed of a thinly schistose subargillaceous limestone at the upper surface of the upper magnesian, called shingle rock by some miners. Layers of shale occur occasionally through the whole extent of the series; sometimes distinct; sometimes firmly attached as a coating to the layers of the limestone. The original color of these is generally blue, but they are often stained green or yellow. They are usually found decomposed to clay in the openings, and are then called, in some places, clay randoms, and are regarded as useful guides in determining the position of the miner. In the lower bed of this rock, layers occur of a very thin black or dark brown shale, more or less bituminous, accompanying particularly the green and brown rock openings at Mineral Point and between Benton and Shullsburg. Thin fossiliferous layers are also met with throughout the series, but most frequently in the lower part. The thicker bedded rock usually contains but very few fossils, and those of large size comparatively, while the thin fossiliferous layers abound in them, and those of small size and usually delicate texture. Some layers are found chiefly composed of minute fossils and concretions. The distinctive fossil of the entire series is the coral, called honey-comb or sun-flower, (Coscinopora.) I have observed it in all the beds of this series, but in none of the other limestones.

The upper bed of this series contains few or no flints, and is usually much thicker than either of the lower beds, and indeed, where it has suffered no denudation, is at least equal in thickness to the two lower combined. The middle bed abounds in flints, arranged in regular layers of nodules, usually white or light grey, but sometimes dark grey or black. The lower bed usually contains but few flints, but these are sometimes more abundant, particularly in the openings.

The character of the lower bed has not appeared as uniform as that of the two higher beds. Like the upper bed, it sometimes is light grey or bluish and compact, and is then valuable for building, when not too much jointed; but it is more often much traversed by argillaceous seams, separating or marking the surface of the layers. This bed is farther characterized by two peculiar rocks, known as the brown or black rock and the green rock, which occupy corresponding positions, but are usually found in different sections of the mineral district. On the Mississippi and Fever river, the brown rock is generally found connected with the openings in the lower bed, and contains more or less calcareous spar (tiff) disseminated through it. The green rock is found in a similar position in the northern and northeastern diggings. The original color of these rocks is bluish, but they have derived their present tint from the decomposition of iron pyrites disseminated through them. The brown rock is of a more or less deep red brown color, usually pervading it uniformly, and from its peculiar tint, was called the chocolate-brown rock by Locke (Owen's 1st Rep.) The green rock is usually less uniformly stained, sometimes only on its seams, and apparently derives its color from the green hydrate of iron. Thin layers often occur in this bed, composed chiefly of flattened fucoidal concretions, but rarely containing any fossils. Similar layers are occasionally found in the higher beds.

Bars of a hard blue limestone often traverse the upper magnesian, in its different portions, more usually in a horizontal position, like beds, but sometimes in a vertical position, like veins. They are more or less intersected by iron pyrites, and are appa

rently connected with mineral deposits, to which they have an important relation. They often interrupt the progress of mineral veins, and are then said, by the miners, to cut off the mineral ; whence the opinion has prevailed that the blue limestone cuts off the mineral, an opinion erroneously transferred to the blue limestone of Owen, to which it has properly no reference. This subject will be farther discussed in, connexion with that of mineral deposits and veins.

BLUE LIMESTONE.

The Blue Limestone series includes the blue limestone and the buff limestone of Owen's first report. These both evidently belong to the same series; the first including the two upper beds, the second the lower bed, already indicated. The three beds, of which the series is composed, are of nearly equal thickness.

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The upper bed is chiefly composed of thinner more fossiliferous, layers, between which are interposed some thicker and less fossiliferous. Some of the layers are almost entirely composed of fossils, and in some instances are subject to decomposition, leaving the fossils loose and entire. Thin layers of bluish shale alternate with the layers of limestone, and are often found decomposed to a soft clay, usually stained yellow or green, particularly in the openings. The layers of limestone are marked by a peculiar parallel or laminated structure, distinct from that of the upper magnesian, and are partly light grey and compact, furnishing the best lime, and partly blue and more distinctly parallel in their structure, and appa rently subargillaceous. Some of the latter kind have been found to furnish a good hydraulic cement. This bed is usually overlaid by a bed of brown rock, in thin layers, and breaking in small jointed fragments, with more or less calcareous spar disseminated, but with few or no fossils. It is interposed, in the northern dis tricts, between the green rock and the blue limestone, and may bel considered as the lowest member of the upper magnesian. In some instances, a bed of blue shale, decomposing into a soft clay in the openings, is interposed between the upper magnesian and the blue limestone.