GEOLOGY

    The soil of Iowa may be separated into three general divisions, which not only possess different physical characters, but also differ in the mode of their origin. These are drift, bluff and alluvial, and belong respectively to the deposits bearing the same names. The drift occupies a much larger part of the surface of the State than both the others. The bluff has the next greatest area of surface, and the alluvial least.

    All soil is disintegrated rock. The drift deposit of Iowa was derived, to a considerable extent, from the rocks of Minnesota; but the greater part of Iowa drift was derived from its own rocks, much of which has been transported but a short distance. In general terms the constant component element of the drift soil is that portion which was transported from the north, while the inconstant elements are those portions which were derived from the adjacent or underlying strata. For example, in Western Iowa, wherever that cretaceous formation known as the Nishnabotany sandstone exists, the soil contains more sand than elsewhere. The same may be said of the soil of some parts of the State occupied by the lower coal measures, the sandstones and sandy shales of that formation furnishing the sand.

    In Northwestern Iowa, the drift contains more sand and gravel than elsewhere. This sand and gravel was, doubtless, derived from the

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cretaceous rocks that now do, or formerly did, exist there, and also in part from the conglomerate and pudding-stone beds of the Sioux quartzite.

    In Southern Iowa, the soil is frequently stiff and clayey. This preponderating clay is doubtless derived from the clayey and shaly beds which alternate with the limestones of that region.

    The bluff soil is that which rests upon, and constitutes a part of, the bluff deposit. It is found only in the western part of the State, and adjacent to the Missouri River. Although it contains less than one per cent. of clay in its composition, it is in no respect inferior to the best drift soil.

    The alluvial soil is that of the flood plains of the river valleys, or bottom lands. That which is periodically flooded by the rivers is of little value for agricultural purposes; but a large part of it is entirely above the reach of the highest floods, and is very productive.

    The stratified rocks of Iowa range from the Azoic to the Mesozoic, inclusive; but the greater portion of the surface of the State is occupied by those of the Palæzoic age. The table below will show each of these formations in their orders:

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rock may be quarried in a few rare cases, but usually it cannot be secured in dry forms except that into which it naturally cracks, and the tendency is to angular pieces. It is absolutely indestructible.

LOWER SILURIAN SYSTEM

PRIMORIDAL GROUP

    Potsdam Sandstone.—This formation is exposed only in a small portion of the northeastern portion of the State. It is only to be seen in the bases of the bluffs and steep valley sides which border the river there. It may be seen underlying the lower magnesian limestone, St. Peter's sandstone and Trenton limestone, in their regular order, along the bluffs of the Mississippi from the northern boundary of the State as far south as Guttenburg, along the Upper Iowa for a distance of about twenty miles form its mouth, and along the Upper Iowa for a distance of about twenty miles from its mouth, and along a few of the streams which empty into the Mississippi in Allamakee County.

    It is nearly valueless for economic purposes.

    No fossils have been discovered in this formation in Iowa.

    Lower Magnesium Limestone—This formation has but little greater geographical extent in Iowa than the Potsdam sandstone. It lacks a uniformity of texture and stratification, owing to which it is not generally valuable for building purposes.

    The only fossils found in this formation in Iowa are a few traces of crinoids, near McGregor.

    St. Peter's Sandstone—This formation is remarkably uniform in thickness throughout its known geographical extent; and it is evident it occupies a large portion of the northern half of Allamakee County, immediately beneath the drift.

TRENTON GROUP

    Trenton Limestone—With the exception of this, all the limestones of both Upper and Lower Silurian age in Iowa are magnesian limestones—nearly pure dolomites. This formation occupies large portions of Winneshiek and Allamakee Counties and a portion of Clayton. The greater part of it is useless for economic purposes, yet there are in some places compact and evenly bedded layers, which afford fine material for window caps and sills.

    In this formation, fossils are abundant, so much so that, in some places, the rock is made up of a mass of shells, corals and fragments of tribolites, cemented by calcareous material into a solid rock. Some of these fossils are new to science and peculiar to Iowa.

    The Galena Limestone—This is the upper formation of the Trenton group. It seldom exceeds twelve miles in width, although it is fully one hundred and fifty miles long. The outcrop traverses portions of the counties of Howard, Winneshiek, Allamakee, Fayette, Clayton, Dubuque and Jackson. It exhibits its greatest development in Dubuque County. It is nearly a pure dolomite, with a slight admixture of siliceous matter. It is usually unfit for dressing,

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though sometimes near the top of the bed good blocks for dressing are found. This formation is the source of the lead ore of the Dubuque lead mines. The lead region proper is confined to an area of about fifteen miles square in the vicinity of Dubuque. The ore occurs in vertical fissures, which traverse the rock at regular intervals from east to west; some is found in those which have a north and south direction. The ore is mostly that known as Galena, or sulphuret of lead, very small quantities only of the carbonate being found with it.

CINCINNATI GROUP

    Maquoketa Shales—The surface occupied by this formation is singularly long and narrow, seldom reaching more than a mile or two in width, but more than a hundred miles in length. Its most southerly exposure is in the bluffs of the Mississippi near Bellevue, in Jackson County, and the most northerly yet recognized is in the western part of Winneshiek County. The whole formation is largely composed of bluish and brownish shales, sometimes slightly arenaceous, sometimes calcareous, which weather into a tenacious clay upon the surface, and the soil derived from it is usually stiff and clayey. Its economic value is very slight.

    Several species of fossils which characterize the Cincinnati group are found in the Maquoketa shales; but they contain a larger number that have been found anywhere else than in these shales in Iowa, and their distinct faunal characteristics seem to warrant the separation of the Maquoketa shales as a distinct formation from any others of the group.

UPPER SILURIAN SYSTEM

NIAGRARA GROUP

    Niagara Limestone—The area occupied by the Niagara limestone is nearly one hundred and sixty miles long from north to south, and forty and fifty miles wide.

    This formation is entirely a magnesian limestone, with in some places a considerable proportion of silicious matter in the form of chert or coarse flint. A large part of it is evenly bedded, and probably affords the best and greatest amount of quarry rock in the State. The quarries at Anamosa, LeClaire and Farley are all opened in this formation.

DEVONIAN SYSTEM

HAMILTON GROUP

    Hamilton Limestone—The area of surface occupied by the Hamilton limestone and shales is fully as great as those by all the formations of both Upper and Lower Silurian age in the State. It is nearly two hundred miles long and from forty to fifty miles broad. The general trend is northwestward and southeastward.

    Although a large part of the material of this formation is practically quite worthless, yet other portions are valuable for economic purposes; and having a

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large geographical extent in the State, is one of the most important formations, in a practical point of view. At Waverly, Bremer County, its value for the production of hydraulic lime has been practically demonstrated. The heavier and more uniform magnesian beds furnish material for bridge piers and other material requiring strength and durability.

    All the Devonian strata of Iowa evidently belong to a single epoch, and referable to the Hamilton, as recognized by New York geologists.

    The most conspicuous and characteristic fossils of this formation are brachiopod, mollusks and corals. The coral Acervularia Davidsoni occurs near Iowa City, and is known as "Iowa City Marble," and "bird's-eye marble."

CARBONIFEROUS SYSTEM

    Of the three groups of formations that constitute the carboniferous system, viz., the subcarboniferous, coal measures and permian, only the first two are found in Iowa.

SUBCARBONIFEROUS GROUP

    The area of the surface occupied by this group is very large. Its eastern border passes from the northeastern part of Winnebago County, with considerable directness in a southeasterly direction to the northern part of Washington County. Here it makes a broad and direct bend nearly eastward, striking the Mississippi River at Muscatine. The southern and western boundary is to a considerable extent the same as that which separates it from the coal field. From the southern part of Pocahontas County it passes southeast to Fort Dodge, thence to Webster City, thence to a point three or four miles northeast of Eldora, in Hardin County, thence southward to the middle of the north line of Jasper County, thence southeastward to Sigourney, in Keokuk County, thence to the northeastern corner of Jefferson County, thence sweeping a few miles eastward to the southeast corner of Van Buren County. Its area is nearly two hundred and fifty miles long, and from twenty to fifty miles wide.

    The Kinderhook Beds—The most southerly exposure of these beds is near the mouth of Skunk River, in Des Moines County. The most northerly now known is in the eastern part of Pocahontas County, more than two hundred miles distant. The principal exposures of this formation are along the bluffs which border the Mississippi and Skunk Rivers, where they from the eastern and northern boundary of Des Moines County, along English River, in Washington County; along the Iowa River, in Tama, Marshall, Hamlin and Franklin Counties; and along the Des Moines River, in Humboldt County.

   The economic value of this formation is very considerable, particularly in the northern portion of the region it occupies. In Pocahontas and Humboldt Counties it is almost invaluable, as no other stone except a few boulders are found here. At Iowa Falls the lower division is very good for building purposes. In Marshall County all the limestone to be obtained comes from this formation, and the quarries near LeGrand are very valuable. At this point

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some of the layers are finely veined with peroxide of iron, and are wrought into ornamental and useful objects.

    In Tama County, the oolitic member is well exposed, where it is manufactured into lime. It is not valuable for building, as upon exposure to atmosphere and frost, it crumbles to pieces.

    The remains of fishes are the only fossils yet discovered in this formation that can be referred to the sub-kingdom VERTEBRATA; and so far as yet recognized, they all belong to the order selachians.

    Of ARTICULATES, only two species have been recognized, both of which belong to the genus phillipsia.

    The sub-kingdom MOLLUSCA is largely represented.

    The RADIATA are represented by a few crinoids, usually found in a very imperfect condition. The sub-kingdom is also represented by corals.

    The prominent feature in the life of this epoch was molluscan; so much so in fact as to overshadow all other branches of the animal kingdom. The prevailing classes are: lamellibranchiates, in the more arenaceous portions; and brachiopods, in the more calcareous portions.

    No remains of vegetation have been detected in any of the strata of this formation.

    The Burlington Limestone—This formation consists of two distinct calcareous divisions, which are separated by a series of silicious beds. Both divisions are eminently crinoidal.

    The southerly dip of the Iowa rocks carries the Burlington limestone down, so that it is seen for the last time in this State in the valley of Skunk River, near the southern boundary of Des Moines County. The most northerly point at which it has been recognized is in the northern part of Washington County. It probably exists as far north as Marshall County.

    This formation affords much valuable material for economic purposes. The upper division furnishes excellent common quarry rock.

    The great abundance and variety of its fossils—crinoids—now known to be more than three hundred, have justly attracted the attention of geologists in all parts of the world.

    The only remains of vertebrates discovered in this formation are those of fishes, and consist of teeth and spines; bone of bony fishes, like those most common at the present day, are found in these rocks. On Buffington Creek, in Louisa County, is a stratum in an exposure so fully charged with these remains that it might with propriety be called bone breccia.

    Remains of articulates are rare in this formation. So far as yet discovered, they are confined to two species of tribolites of the genus phillipsia.

    Fossil shells are very common.

    The two lowest classes of the sub-kingdom radiata are represented in the genera zaphrentis, amplexus and syringapora, while the highest class—echinoderms—are found in most extraordinary profusion.

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    The Keokuk Limestone