The Keokuk Limestone.-It is only in the four counties of Lee, Van Buren, Henry and Des Moines that this formation is to be seen.

In some localities the upper silicious portion of this formation is known as the Geode bed. It is not recognizable in the northern portion of the formation, nor in connection with it where it is exposed, about eighty miles below Keokuk.

The geodes of the Geode bed are more or less spherical masses of silex, usually hollow and lined with crystals of quartz. The outer crust is rough and unsightly, but the crystals which stud the interior are often very beautiful. They vary in size from the size of a walnut to a foot in diameter.

The economic value of this formation is very great. Large quantities of its stone have been used in the finest structures in the State, among which are the post offices at Dubuque and Des Moines. The principal quarries are along the

banks of the Mississippi, from Keokuk to Nauvoo.

The only vertebrate fossils found in the formation are fishes, all belonging to the order selachians, some of which indicate that their owners reached a length of twenty-five or thirty feet.

Of the articulates, only two species of the genus phillipsia have been found in this formation.

Of the mollusks, no cephalopods have yet been recognized in this formation in this State; gasteropods are rare; brachiopods and polyzoans are quite abundant. Of radiates, corals of genera zaphrentes, amplexus and aulopera are found, but crinoids are most abundant.

Of the low forms of animal life, the protozoans, a small fossil related to the sponges, is found in this formation in small numbers.

The St. Louis Limestone.—This is the uppermost of the subcarboniferous group in Iowa. The superficial area it occupies is comparatively small, because it consists of long, narrow strips, yet its extent is very great. It is first seen resting on the geode division of the Keokuk limestone, near Keokuk. Proceeding northward, it forms a narrow border along the edge of the coal fields in Lee, Des Moines, Henry, Jefferson, Washington, Keokuk and Mahaska Counties. It is then lost sight of until it appears again in the banks of Boone River, where it again passes out of view under the coal measures until it is next seen in the banks of the Des Moines, near Fort Dodge. As it exists in Iowa, it consists of three tolerably distinct subdivisions—the magnesian, arenaceous and calcareous.

The upper division furnishes excellent material for quicklime, and when quarries are well opened, as in the northwestern part of Van Buren County, large blocks are obtained. The sandstone, or middle division, is of little economic value. The lower or magnesian division furnishes a valuable and durable stone, exposures of which are found on Lick Creek, in Van Buren County, and on Long Creek, seven miles west of Burlington.

Of the fossils of this formation, the vertebrates are represented only by the remains of fish, belonging to the two orders, selachians and ganoids. The

articulates are represented by one species of the trilobite, genus phillipsia, and two ostracoid, genera, cythre and beyricia. The mollusks distinguish this formation more than any other branch of the animal kingdom. Radiates are exceedingly rare, showing a marked contrast between this formation and the two preceding it.

The rocks of the subcarboniferous period have in other countries, and in other parts of our own country, furnished valuable minerals, and even coal, but. in Iowa the economic value is confined to its stone alone.

The Lower Silurian, Upper Silurian and Devonian rocks of Iowa are largely composed of limestone. Magnesia also enters largely into the subcarboniferous group. With the completion of the St. Louis limestone, the production of the magnesian limestone seems to have ceased among the rocks of Iowa.

Although the Devonian age has been called the age of fishes, yet so far as Iowa is concerned, the rocks of no period can compare with the subcarboniferous in the abundance and variety of the fish remains, and, for this reason, the Burlington and Keokuk limestones will in the future become more famous among geologists, perhaps, than any other formations in North America.

It will be seen that the Chester limestone is omitted from the subcarboniferous group, and which completes the full geological series. It is probable the whole surface of Iowa was above the sea during the time of the formation of the Chester limestone to the southward about one hundred miles.

At the close of the epoch of the Chester limestone, the shallow seas in which the lower coal measures were formed again occupied the land, extending almost as far north as that sea had done in which the Kinderhook beds were formed, and to the northeastward its deposits extended beyond the subcarboniferous groups, outlines of which are found upon the next, or Devonian rock.

THE COAL-MEASURE GROUP.

The coal-measure group of Iowa is properly divided into three formations, viz., the lower, middle and upper coal measures, each having a vertical thickness of about two hundred feet.

A line drawn upon the map of Iowa as follows, will represent the eastern and northern boundaries of the coal fields of the State: Commencing at the southeast corner of Van Buren County, carry the line to the northeast corner of Jefferson County by a slight easterly curve through the western portions of Lee and Henry Counties. Produce this line until it reaches a point six or eight miles northward from the one last named, and then carry it northwestward, keeping it at about the same distance to the northward of Skunk River and its north branch that it had at first, until it reaches the southern boundary of Marshall County, a little west of its center. Then carry it to a point

three or four miles northeast from Eldora, in Hardin County; thence westward to a point a little north of Webster City, in Hamilton County; and thence further westward to a point a little north of Fort Dodge, in Webster County.

Lower Coal Measures.-In consequence of the recedence to the southward of the borders of the middle and upper coal measures, the lower coal measures alone exist to the eastward and northward of Des Moines River. They also occupy a large area westward and southward of that river, but their southerly dip passes them below the middle coal measures at no great distance from the river.

No other formation in the whole State possesses the economic value of the lower coal measures. The clay that underlies almost every bed of coal furnishes a large amount of material for potters' use. The sandstone of these measures is usually soft and unfit, but in some places, as near Red Rock, in Marion County, blocks of large dimensions are obtained which make good building material, samples of which can be seen in the State Arsenal, at Des Moines. On the whole, that portion of the State occupied by the lower coal measures, is not well supplied with stone.

But few fossils have been found in any of the strata of the lower coal measures, but such animal remains as have been found are without exception of marine origin.

Of fossil plants found in these measures, all probably belong to the class acrogens. Specimens of calamites, and several species of ferns, are found in all of the coal measures, but the genus lepidodendron seems not to have existed later than the epoch of the middle coal measures.

Middle Coal Measures.-This formation within the State of Iowa occupies a narrow belt of territory in the southern central portion of the State, embracing a superficial area of about fourteen hundred square miles. The counties. more or less underlaid by this formation are Guthrie, Dallas, Polk, Madison, Warren, Clarke, Lucas, Monroe, Wayne and Appanoose.

This formation is composed of alternating beds of clay, sandstone and limestone, the clays or shales constituting the bulk of the formation, the limestone occurring in their bands, the lithological peculiarities of which offer many contrasts to the limestones of the upper and lower coal measures. The formation is also characterized by regular wave-like undulations, with a parallelism which indicates a widespread disturbance, though no dislocation of the strata have been discovered.

Generally speaking, few species of fossils occur in these beds. Some of the shales and sandstone have afforded a few imperfectly preserved land plants— three or four species of ferns, belonging to the genera. Some of the carbonif erous shales afford beautiful specimens of what appear to have been sea-weeds. Radiates are represented by corals. The mollusks are most numerously represented. Trilobites and ostracoids are the only remains known of articulates.

Vertebrates are only known by the remains of salachians, or sharks, and ganoids.

Upper Coal Measures.-The area occupied by this formation in Iowa is very great, comprising thirteen whole counties, in the southwestern part of the State. It adjoins by its northern and eastern boundaries the area occupied by the middle coal measures.

The prominent lithological features of this formation are its limestones, yet it contains a considerable proportion of shales and sandstones. Although it is known by the name of upper coal measures, it contains but a single bed of coal, and that only about twenty inches in maximum thickness.

The limestone exposed in this formation furnishes good material for building as in Madison and Fremont Counties. The sandstones are quite worthless. No beds of clay for potter's use are found in the whole formation.

The fossils in this formation are much more numerous than in either the middle or lower coal measures. The vertebrates are represented by the fishes of the orders selachians and ganoids. The articulates are represented by the trilobites and ostracoids. Mollusks are represented by the classes cephalapoda, gasteropoda, lamelli, branchiata, brachiapoda and polyzoa. Radiates are more numerous than in the lower and middle coal measures. Protogoans are represented in the greatest abundance, some layers of limestone being almost entirely composed of their small fusiform shells.

CRETACEOUS SYSTEM.

There being no rocks, in Iowa, of permian, triassic or jurassic age, the next strata in the geological series are of the cretaceous age. They are found in the western half of the State, and do not dip, as do all the other formations upon which they rest, to the southward and westward, but have a general dip of their own to the north of westward, which, however, is very slight. Although the actual exposures of cretaceous rocks are few in Iowa, there is reason to believe that nearly all the western half of the State was originally occupied by them; but being very friable, they have been removed by denudation, which has taken place at two separate periods. The first period was during its elevation from the cretaceous sea, and during the long tertiary age that passed between the time of that elevation and the commencement of the glacial epoch. The second period was during the glacial epoch, when the ice produced their entire removal over considerable areas.

It is difficult to indicate the exact boundaries of these rocks; the following will approximate the outlines of the area:

From the northeast corner to the southwest corner of Kossuth County; thence to the southeast corner of Guthrie County; thence to the southeast corner of Cass County; thence to the middle of the south boundary of Montgomery County; thence to the middle of the north boundary of Pottawattamie County; thence to the middle of the south boundary of Woodbury County;

thence to Sergeant's bluffs; up the Missouri and Big Sioux Rivers to the northwest corner of the State; eastward along the State line to the place of beginning.

All the cretaceous rocks in Iowa are a part of the same deposits farther up the Missouri River, and in reality form their eastern boundary.

Nishnabotany Sandstone. This rock has the most easterly and southerly extent of the cretaceous deposits of Iowa, reaching the southeastern part of Guthrie County and the southern part of Montgomery County. To the northward, it passes beneath the Woodbury sandstones and shales, the latter passing beneath the inoceramus, or chalky, beds. This sandstone is, with few exceptions, almost valueless for economic purposes.

The only fossils found in this formation are a few fragments of angiospermous leaves.

Woodbury Sandstones and Shales.-These strata rest upon the Nishnabotany sandstone, and have not been observed outside of Woodbury County, hence their name. Their principal exposure is at Sergeant's Bluffs, seven miles below Sioux City.

This rock has no value except for purposes of common masonry.

Fossil remains are rare. Detached scales of a lepidoginoid species have been detected, but no other vertebrate remains. Of remains of vegetation, leaves of salix meekii and sassafras cretaceum have been occasionally found.

Inoceramus Beds.-These beds rest upon the Woodbury sandstones and shales. They have not been observed in Iowa, except in the bluffs which border the Big Sioux River in Woodbury and Plymouth Counties. They are composed almost entirely of calcareous material, the upper portion of which is extensively used for lime. No building material is to be obtained from these beds; and the only value they possess, except lime, are the marls, which at some time may be useful on the soil of the adjacent region.

The only vertebrate remains found in the cretaceous rocks are the fishes. Those in the inoceramus beds of Iowa are two species of squoloid selachians, or cestratront, and three genera of teliosts. Molluscan remains are rare.

PEAT.

Extensive beds of peat exist in Northern Middle Iowa, which, it is estimated, contain the following areas:

Several other counties contain peat beds, but the character of the peat is inferior to that in the northern part of the State. The character of the peat