submitted a report to the Governor in two large volumes.

From these reports we derive a pretty thorough knowledge of the geological characteristics in all portions of the State.

In the classification of Iowa rocks, State Geologist White adopted the following definitions:

The term "formation" is restricted to such assemblages of strata as have been formed within a geological epoch; the term "group," to such natural groups of formation as were not formed within a geological period; and the term "system," to such series of groups as were each formed within a geological age.

The terms used in this arrangement may be referred to two categoriesone applicable to geological objects, and the other to geological time. Thus: Formations constitute Groups; groups constitute Systems; Epochs constitute Periods; periods constitue Ages.

In accordance with this arrangement the classification of Iowa rocks may be seen at a glance in the following table constructed by Dr. White:

Huronian Group. The Sioux Quartzite Formation in this Group is found exposed in natural ledges only on a few acres in the northwest corner of the State. The exposures in Iowa are principally upon the banks of the Big Sioux river, for which reason the specific name of Sioux Quartzite is given to it. It is an intensely hard rock, breaking with a splintery fracture, and a color varying in different localities from a bright to a deep red. Although it is so compact and hard the grains of sand of which it was originally composed are yet distinctly to be seen, and even the ripple marks upon its bedding surfaces are sometimes found as distinct as they were when the rock was a mass of incoherent sand in the shallow waters in which it was accumulated. The lines of stratification are also quite distinct, but they are not usually sufficiently definite to cause the mass to divide into numerous layers. It has, however, a great tendency to break up by vertical cracks

and fissures into small angular, blocks. The process of metamorphism has been so complete throughout the whole formation that the rock is almost everywhere of uniform texture, and its color also being so nearly uniform there is no difficulty in identifying it wherever it may be seen.

In a few rare cases this rock may be quarried readily, as the layers are easily separated, but usually it is so compact throughout that it is quarried with the greatest difficulty into any forms except those into which it naturally eracks. It has a great tendency, however, upon its natural exposures, to break up by vertical fissures and cracks into angular blocks of convenient size for handling. Except this tendency to crack into angular pieces, the rock is absolutely indestructible. No traces of fossil remains of any kind have been found in it. As shown by the table its exposure in Iowa is fifty feet in thickness.

LOWER SILURIAN SYSTEM.

Primordial Group.-The Potsdam Sandstone Formation of this Group has a geographical range extending throughout the northern portion of the United States and Canada, and in Iowa reaches a known thickness of about 300 feet, as shown in the table. It forms, however, rather an inconspicuous feature in the geology of Iowa. It is exposed only in a small portion of the northeastern part of the State, and has been brought to view there by the erosion of the river valleys. The base of the formation does not appear anywhere in Iowa, consequently its full thickness is not certainly known, nor is it known certainly that it rests on the Sioux Quartzite. The rock is everywhere soft; usually a very friable sandstone, but sometimes containing some clayey material, and approaching in character a sandy shale. It is nearly valueless for any economic purpose, not being of sufficient hardness to serve even the commonest purposes of masonry. No fossils have been discovered in this formation in Iowa, but in Wisconsin they are found quite abundantly in it.

The Lower Magnesian Limestone Formation has but little greater geographical extent in Iowa than the Potsdam Sandstone has; because, like that formation, it appears only in the bluffs and valley-sides of the same streams. It is a more conspicuous formation, however; because, being a firin rock, it presents bold and often picturesque fronts along the valleys. Its thickness is about 250 feet, and is quite uniform in composition, being a nearly pure buff-colored dolomite. It lacks a uniformity of texture and stratification which causes it to weather into rough and sometimes grotesque shapes, as it stands out in bold relief upon the valley-sides. It is not generally valuable for building purposes, owing to its lack of uniformity in texture and bedding. Some parts of it, however, are selected which serve for such nses at Lansing and McGregor. It has also been used to some extent for making lime, but it is not equal to the Trenton limestone, near Dubuque, for that purpose. The only fossils that have been found in this formation in Iowa, are, so far as known, a few traces of the stems of Crinoids found near McGregor.

The St. Peter's Sandstone Formation is remarkably uniform in thickness throughout its known geographical extent. It is a clean grit, light colored, very friable rock; so pure in its silicious compostion that it is probable some portions of it may be found suitable for the manufacture of glass. pies the surface of a large portion of the north half of Allemakee county, immediately beneath the drift, and it is also exposed a couple of miles

It occu

below McGregor, where it is much colored by oxide of iron. It contains no fossils.

Trenton Group.-The lower formation of this group is known as the 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. The rocks of this formation also contain much magnesia, but a large part of it is composed of bluish compact common limestone. It occupies large portions of both Winneshiek and Allamakee counties, together with a portion of Clayton. Its thickness as seen along the bluffs of the Mississippi is about eighty feet, but in Winneshiek county we find the thickness is increased to upward of 200 feet. The greater part of this formation is worthless for economic purposes, but enough of it is suitable for building purposes and for lime to meet the wants of the inhabitants. The worthless portions of the formation consists of clayey shales and shaly limestone. Fossils are abundant in this formation. In some places the rock is made up of a mass of shells, corals, and fragments of trilobites, together with other animal remains, cemented by calcareous matter into compact form.

The upper portion of the Trenton Group, known as the Galena Limestone Formation, occupies a narrow strip of country, seldom exceeding 12 miles in width, but it is fully 150 miles long. It is about 250 feet thick in the vicinity of Dubuque, but diminishes in thickness as it extends northwest, so that it does not probably exceed 100 feet where it crosses the northern boundary of the State. The outcrop of this formation traverses portions of the counties of Howard, Winneshiek, Allamakee, Fayette, Clayton, Dubuque, and Jackson. It exhibits its greatest development in Dubuque county. It is not very uniform in texture, which causes it to decompose unequally, and consequently to present interesting forms in the abrupt bluffs of it, which border the valleys. It is usually unfit for dressing, but affords good enough stone for common masonry. It is the source of the lead ore of the Dubuque lead mines. The full thickness of this formation at Dubuque is 250 feet. Fossils are rare in it.

Cincinnati Group.-The Maquoketa Shale Formation of this group, socailed by Dr. White, is synonymous with the Hudson River Shales, of Prof. Hall. It is comprised within a long and narrow area, seldom reaching more than a mile or two in width, but more than a hundred miles long, in the State. Its most southerly exposure is in the bluffs of the Mississippi river, near Bellevue, in Jackson county, and the most northerly one yet recognized is in the western part of Winneshiek county. The whole formation is largely composed of bluish and brownish shales. Its economic value is very slight, as it is wholly composed of fragmentary materials. The fossils contained in this formation, together with its position in relation to the underlying and overlying formations, leave no doubt as to the propriety of referring it to the same geological period as that in which the rocks at Cincinnati, Ohio, were formed. Several species of fossils which characterize the Cincinnati group are found in the Maquoketa Shales, but they contain a large number of species that have been found nowhere else than in these shales in Iowa, and it is the opinion of Dr.White that the occurrence of these distinct fossils in the Iowa formation would seem to warrant the separation of the Maquoketa Shales as a distinct formation from any others of the group, and that its true position is probably at the base of the Cincinnati group.

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 important 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