UPPER SILURIAN SYSTEM.

Niagara Group.-The area occupied by the Niagara limestone Formation is nearly 160 miles from north to south, and between 40 and 50 miles wide in its widest part. At its narrowest part, which is near its northern limit in Iowa, it is not more than four or five miles wide. This formation is entirely magnesian limestone, with, in some places, a considerable proportion of silicious matter in the form of chert or coarse flint. Some of the lower portions resemble both the Galena and Lower Magnesian Limestones, having the same want of uniformity of texture and bedding. It affords, however, a great amount of excellent quarry rock. The quarries at Anamosa, in Jones county, are remarkable for the uniformity of the bedding of its strata. Wherever this rock is exposed there is always an abundance of material for common masonry and other purposes. In some places excellent lime is made from it.

DEVONIAN SYSTEM.

Hamilton Group.-The Hamilton Limestone and Shales Formation occupies an area of surface as great as those occupied by all the formations of both Lower and Upper Silurian age in the State. The limestones of the Devonian age are composed in part of magnesian strata, and in part of common limestone. A large part of the material of this formation is quite worthless, yet other portions are very valuable for several economic purposes. Having a very large geographical extent in Iowa, it constitutes one of the most im portant formations. Wherever any part of this formation is exposed, the common limestone portions exist in sufficient quantity to furnish abundant material for common lime of excellent quality, as well as good stone for common masonry. Some of the beds furnish excellent material for dressed stone, for all works requiring strength and durability. The most conspicuous and characteristic fossils of this formation are brachipod mollusks and corals.

CARBONIFEROUS SYSTEM.

The Sub-Carboniferous Group.-This group occupies a very large surface in Iowa. Its eastern border passes from the northeastern portion of Winnebago county 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 the city of Muscatine. The southern and western boundary of the area is to a considerable extent the same as that which separates it from the coalfield. From the southern part of Pocahontas county, it passes southeastward 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 northeast corner of Jefferson county, and thence, by sweeping a few miles eastward to the southeast corner of Van Buren county. The area as thus defined, is nearly 250 miles long, and from 20 to 40 miles wide. The general southerly and westerly dip has carried the strata of the group beneath the lower coalmeasure along the line last designated, but after passing beneath the latter strata for a distance of from 15 to 20 miles, they appear again in the valley of the Des Moines river, where they have been bared by the erosion of that valley.

The Kinderhook Beds, the lowest Formation of the sub-carboniferous group,

presents its principal exposures along the bluffs which border the Mississippi and Skunk rivers, where they form the eastern and northern boundary of Des Moines county; along English river in Washington county; along Iowa river in Tama, Marshall, Hardin and Franklin counties, and along the Des Moines river in Humboldt county. The southern part of the formation in Iowa has the best development of all in distinguishing characteristics, but the width of area it occupies is much greater in its northern part, reaching a maximum width of eighty miles. The Kinderhook formation has considerable economic value, particularly in the northern portion of the region it occupies. The stone which it furnishes is of practical value. There are no exposures of stone of any other kind in Pocahontas, Humboldt and some other counties embraced in the area occupied by it, and therefore it is of very great value in such places for building material. It may be manufactured into excellent lime. The quarries in Marshall county and at Le Grand are of this formation; also the oolitic limestone in Tama county. This oolitic limestone is manufactured into a good quality of lime. The principal fossils appearing in this formation are the remains of fishes; no remains of vegetation have as yet been detected. The fossils in this formation, so far as Iowa is concerned, are far more numerous in the southern than in the northern part.

The Burlington Limestone is the next Formation in this group above the Kinderhook Beds, the latter passing gradually into the Burlington Limestone. This formation consists of two distinct calcareous divisions, which are separated by a series of silicious beds. The existence of these silicious beds suggests the propriety of regarding the Burlington Limestone as really two distinct formations. This is strengthened also by some well marked palaeontological differences, especially in the crinoidal remains. 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. Northward of Burlington it is found frequently exposed in the bluffs of the Mississippi and Iowa rivers in the counties of Des Moines and Louisa, and along some of the smaller streams in the same region. Burlington Limestone forms a good building material; good lime may also be made from it, and especially from the upper division. Geologists have given to this formation the name of Burlington Limestone because its peculiar characteristics are best shown at the city of Burlington, Iowa. The great abundance and variety of its characteristic fossils crinoids-have attracted the attention of geologists and naturalists generally. The only remains of vertebrates reported as being found in it are those of fishes. Remains of articulates are rare in it, and confined to two species of trilobites. Fossil shells are common but not so abundant as in some of the other formations of the Sub-Carboniferous Group.

The Keokuk Limestone is the next Formation in this group above the Burlington Limestone. In Iowa it consists of about fifty feet in maximum thickness. It is a grayish limestone, having usually a blueish tinge. It occupies in Iowa a more limited area than any other formation of the subcarboniferous group. It is well developed and largely exposed at the city of Keokuk. It is synonymous with the Lower Archimedes Limestone of Owen and other geologists. The most northerly point at which it has been recognized is in the northern part of Des Moines county, where it is quite thinned out. It is only in the counties of Lee, Van Buren, Henry and Des Moines that the Keokuk Limestone is to be seen; but it rises again and is

seen in the banks of the Mississippi river some seventy-five or eighty miles below Keokuk, presenting there the same characteristics that it has in Iowa. The upper silicious portion of this formation is known as the Geode bed. These geodes are more or less spherical masses of silex, usually hollow and lined with crystals of quartz. The Keokuk Limestone formation is of great economic value, as some of its layers furnish a fine quality of building material. The principal quarries of it are along the Mississippi from Keokuk to Nauvoo, a distance of about fifteen miles. The only vertebrated fossils in it are those of fishes, consisting both of teeth and spines. Some of these are of great size, indicating that their owners probably reached a length of twenty-five or thirty feet. Several species of articulates, mollusks and radiates are also found in this formation. Among the radiates the crinoids. are very abundant, but are not so conspicuous as in the Burlington Limestone. A small number of Protozoans, a low form of animal life, related to sponges, have also been found in the Keokuk Limestone.

The next Formation in the Sub-Carboniferous Group, above the Keokuk Limestone, is what Dr. White calls the St. Louis Limestone, and is synonymous with the Concretionary Limestone of Prof. Owen, and the Warsaw Limestone of Prof. Hall. It is the upper, or highest formation of what Dr. White classifies as the Sub-Carboniferous Group, appearing in Iowa, where the lower coal-measures are usually found resting directly upon it, and where it forms, so to speak, a limestone floor for the coal-bearing formations. To this, however, there are some exceptions. It presents a marked contrast with the coal-bearing strata which rest upon it. This formation occupies a small superficial area in Iowa, because it consists of long narrow strips. Its extent, however, within the State is known to be very great, because it is found at points so distant from each other. Commencing at Keokuk, where it is seen resting on the geode division of the Keokuk limestone, and proceeding northward, it is found forming a narrow border along the edge of the coal-field in Lee, Des Moines, Henry, Jefferson, Washington, Keokuk and Mahaska counties. It is then lost sight of beneath the coal-measure strata and overlying drift until we reach Hamilton county, where it is found in the banks of Boone river with the coal-measures resting upon it, as they do in the counties just named. The next seen of the formation is in the banks of the Des Moines river at and near Fort Dodge. These two last named localities are the most northerly ones at which the formation is exposed, and they are widely isolated from the principal portion of the area it occupies in Iowa; between which area, however, and those northerly points, it appears by a small exposure near Ames, in Story county, in the valley of a small tributary of Skunk river. This formation as it appears in Iowa, consists of three quite distinct sub-divisions-magnisian, arenaceous and calcareous, consisting in the order named of the lower, middle and upper subdivisions of the formation. The upper division furnishes excellent material for quicklime, and in places it is quarried to serve a good purpose for maThe middle division is of little economic value, being usually too soft for practical use. The lower, or magnesian division, furnishes some excellent stone for heavy masonry, and has proved to be very durable. This formation has some well marked fossil characteristics, but they do not stand out with such prominence as some of those in the two preceding formations. The vertibrates, articulates, mollusks, and radiates, are all more or less represented in it. Some slight vegetable remains have also been detected in it. The Coal-measure Group. The formations of this group are divided

into the Lower, Middle, and Upper Coal-measures. Omitting particular reference to the other strata of the Lower Coal-measure, we refer only to the coal which this formation contains. Far the greater part of that indispensible element of material prosperity is contained in the strata of the Lower Coal-measures. Beds are now being mined in this formation that reach to the thickness of seven feet of solid coal. Natural exposures of this formation are few, but coal strata are being mined in a number of localities.

The area occupied by the Middle Coal-measure is smaller than that of either of the others, and constitutes a narrow region between them. The passage of the strata of the Lower with the Middle Coal-measure is not inarked by any well defined line of division.

The area occupied by the Upper Coal-measure formation in Iowa is very great, comprising thirteen whole counties in the southwestern part of the State, together with parts of seven or eight others adjoining. It adjoins by its northern and eastern boundary the area occupied by the Middle Coal-measures. The western and southern limits in Iowa of the Upper Coal-measures are the western and southern boundaries of the State, but the formation extends without interruption far into the States of Missouri, Nebraska and Kansas. It contains but a single bed of true coal, and that very thin. Its principal economic value is confined to its limestone. Wherever this stone is exposed it furnishes good material for masonry, and also for lime. The prevailing color of the limestone is light gray, with usually a tinge of blue. The sandstones of this formation are usually shaly, and quite worthless.

CRETACEOUS SYSTEM.

The Nishnabotany Sandstone.-This formation is well exposed in the valley of the East Nishnabotany river, from which circumstance Dr. White has so named it. It is found as far east as the southeastern part of Guthrie county, and as far south as the southern part of Montgomery county. Το the northwestward it passes beneath the Woodbury sandstones and shales, the latter in turn passing beneath the Inoceramus, or chalky beds. It reaches a maximum thickness in Iowa, so far as known, of about 100 feet, but the exposures usually show a much less thickness. It is a soft sandstone, and, with few exceptions, almost valueless for economic purposes. The most valuable quarries in the strata of this formation, so far as known, are at Lewis, Cass county, and in the northeastern part of Mills county. Several buildings have been constructed of it at Lewis, but with some the color is objectionable, being of a dark brown color. A few fossils have been found in it, being leaves too fragmentary for identification.

The Woodbury Sandstones and Shales.-These are composed of alternating sandstones and shales, as the name implies, and rest upon the Nishinabotany sandstone. They have not been observed outside of the limits of Woodbury county, but they are found there to reach a maximum of about 150 feet. Some layers are firm and compact, but the larger part is impure and shaly. The best of it is suitable for only common masonry, but it furnishes the only material of that kind in that part of the State. Some slight fossil remains have been found in this formation.

The Inoceramus Beds.-These beds constitute the upper formation of the Cretaceous System in Iowa, and have a maximum thickness of about 50 feet. They rest directly upon the Woodbury sandstones and shales. They are

observed nowhere in Iowa except along the bluffs of the Big Sioux river, in Woodbury and Plymouth counties. They are composed of calcareous material, but are not a true, compact limestone. The material of the upper portion is used for lime, the quality of which is equal to that of common limestone. No good building material is obtained from these beds. fossil fish have been found in them.

Some

Above all the formations above-mentioned rests the Post-Tertiary, or Drift deposit, which is more fully mentioned in connection with the Soils of Iowa.

ECONOMIC GEOLOGY.

Coal-Peat-Building Stone-Lime-Lead-Gypsum-Spring and Well Water-ClaysMineral Paint.

COAL.

Every year is adding to our knowledge of, and attesting the importance and value of our vast coal deposits. In some unknown age of the past, long before the history of our race began, Nature by some wise process, made a bountiful provision for the time when, in the order of things, it should become necessary for civilized man to take possession of these broad rich prairies. As an equivalent for the lack of trees, she quietly stored away beneath the soil those wonderful carboniferous treasures for the use and comfort of man at the proper time. The increased demand for coal has in many portions of the State led to improved methods of mining, so that in many counties the business is becoming a lucrative and important one, especially where railroads furnish the means of transportation. The coal field of the State embraces an area of at least 20,000 square miles, and coal is successfully mined in about thirty counties, embracing a territory larger than the State of Massachusetts. Among the most important coal producing counties may be mentioned Appanoose, Boone, Davis, Jefferson, Mahaska, Marion, Monroe, Polk, Van Buren, Wapello, and Webster. Within the last few years many discoveries of new deposits have been made, and counties not previously numbered among the coal counties of the State are now yielding rich returns to the miner. Among these may be mentioned the counties of Boone, Dallas, Hamilton, Hardin, and Webster. A vein of coal of excellent quality, seven feet in thickness, has been opened, and is now being successfully worked, about five miles southeast of Fort Dodge, in Webster county. Large quantities of coal are shipped from that point to Dubuque and the towns along the line of the Dubuque and Sioux City Railroad. A few years ago it was barely known that some coal existed in Boone county, as indicated by exposures along the Des Moines river, and it is only within the last few years that the coal mines of Moingona have furnished the vast supplies shipped along the Chicago and Northwestern Railroad, both east and west. The great productive coal field of Iowa is embraced chiefly within the valley of the Des Moines river and its tributaries, extending up the valley from Lee county nearly to the north line of Webster county. Within the coal field embraced by this valley deep mining is nowhere necessary. The Des Moines and its larger tributaries have generally cut their channels down through the coal ineasure strata.

The coal of Iowa is of the class known as bituminous, and is equal in quality and value to coal of the same class in other parts of the world. The veins which have so far been worked are from three to eight feet in