The middle bed of the blue limestone is composed of more ■niform and thicker bedded very even layers, less abundant in fossils, but presenting some which have not occurred to me in the upper bed, such as trilobites, and the acorn (Streptelasma.) In the western districts, where most distinctly developed, this bed may be divided into three distinct portions: an upper, of a very fine crys talline grain, and of a light grey color, subject to a brown stain in connection with openings; a middle, of a dark grey color, hard and compact, breaking with a smooth conchoidal fracture, and called glass rock, in most of the diggings where it occurs; and a lower, forming a transition to the lower bed, and consisting of alternations of grey compact and bluish parallel seams, firmly connected, the former resembling the glass rock, the latter the prevailing rock of the lower bed. This lower portion is more fossilìferous than the two others, particularly on the surfaces of its layers. This distinction is well marked in Quinby's quarry on the Shullsburg Branch, north of New Diggings. In the most eastern districts, yet examined, this distinction appears less marked, nearly the whole bed being composed of a uniform fine-grained light grey rock, resembling the upper portion. The glass rock is there hardly represented. Nodules of flint occasionally but rarely are found in this middle bed, particularly in its upper fine-grained portion.

The lower bed, corresponding to the buff limestone of Owen, consists chiefly of a thick-bedded even rock, marked by a distinct parallel arrangement, and composed in a great measure of flattened vermiform and fucoidal concretions, most strongly marked on the surfaces of the layers. That these are merely concretions and not organic, appears to me very evident. The same structure is equally remarkable in certain thin subargillaceous layers, observed in upper magnesian, particularly in its lower bed. The same appearance is observable in the transition from the sandstones to the lower magnesian, particularly on the surface of the layers, where marked by argillaceous seams. It would seem to be common whereever there is a combination of lime and alumine. This lower bed furnishes a brown lime, and in some portions of it, a good hydraulic

the

cement, which alone indicates its subargillaceous character. The natural color of this bed is a light blue, but it is very much subject to stain, buff or yellow,* from disseminated iron pyrites. Indeed in some districts, particularly the eastern, the whole series is generally found, at least near the surface, of a yellow color, only a few portions retaining their original blue color. The rock of this lower bed is easily dressed, particularly the middle portion of it, and in some instances is capable of a fine polish, forming, by its concretionary structure, a beautifully clouded marble. Quinby's quarry, above noticed, furnishes fine specimens. The same bed, in the quarry at Monterey (Janesville,) has been used for that purpose, but its effect is injured by small geodic cavities. This lower bed contains comparatively few fossils, particularly in its middle portion. Trilobites have been found in it, as well as in the middle bed. At its junction with the upper sandstone, there is usually a transition from one rock to the other; a number of subsilicious and subargillaceous layers intervening, the former of which are more or less oolitic in their structure.

UPPER SANDSTONE.

The Upper Sandstone forms a bed of a generally uniform character, and of no great thickness, composed usually of fine grains of quartzose sand, very slightly cemented, and consequently very little coherent, often in the interior in the state of loose sand. The surface is generally more or less indurated, but often this harder coat is of very little thickness. The natural color of this rock is white, but it is very subject to stain yellow, red, and sometimes green, from the decomposition of disseminated iron pyrites. These stains are most remarkable on the surface and near the seams, and particularly near the junction of the rock with the adjʊining limestones. At the junction of this rock with the blue limestone above, it is usually coarser grained, and often contains concretions of quartz, sometimes geodic, which have been evidently formed

It has been called, from this circumstance, the buff limestone, but might, with moro propriety, be called the blue and buff limestone.

by chemical action. In this position too, concretions of iron pyrites, or of hematite resulting from its decomposition, are fre quent; the latter often including a portion of the pyrites unchanged. Small nodules or seams of hematite, sometimes with iron pyrites, occur also in this part, filled with grains of quartz of a hyalitic appearance. This layer, which has been apparently so subject to chemical action, is usually of a dark red brown, or of a deep green color, (the latter from the green hydrate of iron,) and Occasionally the adjoining sandstone, to a considerable depth beneath, is more or less stained green from the same cause. This rock is usually too incoherent to answer well for building, although generally sufficiently fine grained and thick-bedded for that purpose. It furnishes, however, a superior sand for mortar, and sometimes so hardens by exposure, as to be useful for building. In some districts, particularly on some of the eastern branches of the East Pecatonica, near the line of Green and Lafayette counties, this rock is composed of thin nearly schistose layers, and its lower part is then more or less filled with minute white calcareous grains, giving it a firmer texture.

LOWER MAGNESIAN.

This rock I have not yet examined through its entire depth, having had an opportunity of viewing it only in its southern and eastern outcrops, on the Platte, Blue, Pecatonica and Sugar rivers, and in a ridge 2-3 miles S. W. of Madison. The greatest depth to which I have yet seen it exposed, is nearly 100 feet, on the Big Platte, in Ellenborough. A thickness of more than 200 feet has been given it, on the Mississippi, by Owen, in his reports. Wherever I have seen it, this rock has presented peculiar external characters, by which it can be readily distinguished from the preceding limestones. Among the distinctive marks which I have observed, the most striking are a eculiar concre tionary nodular structure, and the occurrence of geodes lined with minute crystals of quartz, and of layers of fliut less inter

• Two hundred and twenty-five feet. (Report 1852)

rapted and nodular than in the preceding limestones, either white and abounding in geodes of quartz, or striped red-brow and yellow, resembling a striped jasper, and then more rarely geodic. Fossils are very rare, nor have I yet observed them in this forma

tion.

Where I have had an opportunity of observing it continuously underlying the upper sandstone, on the Blue and Platte rivers, it has presented two distinct beds, an u; per and a lower. The first is composed of a series of alternations of subargillaceous and subsilicious limestones, more or less decomposible, with occasional interposed layers or beds of a puer and harder limestone. The subargillaceous layers sometimes form a marly shale, decomposing into a soft clay, and the subsilicious layers have often a remarkable concretionary structure, and resemble, in their grain at least, the silicious limestone of Fontainebleau. Sometimes layers of near y pure sandstone occur even in the lower part of this bed. Flints, such as I have described, occur in this bed, particularly in the purer limestone, and in connexion with openings; but they have appeared less abundant in this bed than in the lower. From the decomposible character of the greater part of this bed, its surface is generally covered with earth, forming a sloping declivity. The lower bed is composed of a hard and purer thickbedded grey limestone, resembling in its external appearance the corresponding middle bed of the upper magnesian, but distinguished by its structure, and its peculiar flints already noticed. This lower bed has been seen by me on'y in its upper portion. It appears, both on the Blue and Platte rivers, only as a low bluff (10-20 feet high) sinking below the surface. From its character, and particularly the great abundance of flints, it is apparently the middle bed of the entire scries; a lower bed underlying it, corresponding in some degree to the upper bed already described. This, however, I offer only as a conjecture.

LOWER SANDSTONE.

This formation I have not yet had an opportunity of observing in immediate connexion with the overlying stratum (the Lower

Magnesian.) The sandstone in the quarries west of Madison, from which that town is supplied with its material for building, is quite different in its character from the upper sandstone, and is apparently less purely silicious, and consequently less incoherent in its texture. It is overlaid in the quarries, particularly in those on the south (Larkin's,) by subcalcareous and subargillaceous layers, resembling not a little those which occur at the junction of the upper sandstone and the lower magnesian. Concretions of a flinty quartz are found in some of these, resembling similar concretions in the latter situation. From these circumstances, I should rather regard the sandstone in those quarries as belonging to the Lower Sandstone. This is farther rendered probable by the occurrence of those quarries on the north of a ridge, extending along the south side of Dead Lake, occupied by the lower magnesian, while the country to the south of that ridge is occupied by the blue limestone and the underlying upper sandstone.

It is worthy of remark that each of the limestone series admits of a three-fold division, distinct in the three upper series, and at least probable in the lower magnesian. A general character, independent of its fossils, pervades the whole of each series, by which it may be distinguished from the others, while each subdivision or distinct bed has its own distinctive characters. The middle bed in each is distinguished by an abundance of flint or hornstone, arranged in layers conformable to the stratification, either in detached nodules, or more connected. This is less obvious in the middle bed of the blue limestone; still nodules of flint are there of occasional occurrence, particularly in the upper finegrained portion.

Estimates of the thickness of the different strata have been giv. en in former reports; but such can be considered only as approximative, the strata apparently varying considerably in thickness in different localities. It may be considered a moderate estimate to reckon the thickness of the Upper Magnesian at 240 feet (120 feet for the upper, and 60 feet for each of the lower beds ;) that of the