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 form 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

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.

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 plantsthree or four species of ferns, belonging to the genera. Some of the carboniferous 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.