of the formations upon which it rested into the fine material that now constitutes our soil and subsoil, and which now covers those formations like a mantle, so that we seldom see them in place except when they have been bared by the streams while they were cutting out their own valleys. The granite boulders and pebbles so common in all parts of the State were brought down from the north by the same means, for we have no other granitic rocks within our borders, and these with the material ground up by the glaciers were mixed into a heterogenous mass which now occupies the surface every where, and is called "drift" by geologists. Lumps of copper, doubtless from the Lake Superior region, and masses of lead ore probably from the Dubuque lead region are sometimes found in the drift of the different parts of the State, where they happened to be released by the melting ice as the glacial epoch passed away. In Northern Iowa also, quite beyond the limits of our coal- field, lumps of coal have been found in the drift which the inhabitants sometimes regard as an indication of the existence in their neighborhood of a bed of coal. But these lumps were probably brought down by the ice from a thin bed of coal which is believed to exist in Southern Minnesota, and which belongs to rocks of Cretaceous age, and consequently having no connection with the Iowa coal - field. The subject of the drift formation will be found further discussed in the following pages under the heads of "Drift phenomena in Southern Iowa," and "Lakes of Iowa, past and present."

The term "coal-measures" is one introduced from England, and simply means the formation which contains the coal. It was once believed that all the coal of the earth is contained in the formation of that particular geological age, but sabsequent investigation has proved this view to have been incorrect. We have now much evidence tending to prove that all the coal yet found in the far West, as we in Iowa understand those words, is of much later origin than our own, and with which ours has no connection by continuity of formations.

The coal-measures of Iowa are properly separable into upper, middle and lower divisions. The first lies wholly to the southward and westward of the Des Moines River, occupying the surface in the southwestern part of the State over an area equal in extent to that of the State of Vermont. The second doubtless underlies the whole of the first, but it occupies the surface only along its northern and eastern borders, including the greater part of the region of the Raccoon Rivers, which region is more particularly referred to in Mr. St. John's report, on another page. The lower coal-measures occupy the surface of a large area, averaging some fifty miles in width, and about one hundred and seventy-five miles in length, through the middle of which the Des Moines River runs longitudinally in a southeasterly direction, from a little above Ft. Dodge, nearly to Keokuk. Over nearly the whole of this large surface coal may reasonably be sought for. To the northward and eastward of it, it is useless to look for coal except in small "patches" or outliers. These are almost always unproductive of a workable bed of coal; the large outlier extending from Muscatine nearly to Davenport being an exception. To the southward and westward of this region it is believed that coal will be found by sinking shafts to a greater or less depth.

The lower coal-measures contain all the heaviest beds of coal, and by far the

largest part to be found in the State. The middle measures contain some workable beds, but the upper series are believed to contain only one bed of coal, the greatest thickness of which is only twenty inches.

The lower coal measures are made up principally of sandstone, shales, clay and coal, with very little limestone; the middle, of similar materials, but has more limestone, less sandstone, and less coal, while the upper coal measures are composed largely of limestone, which in Madison county alone is found to reach a thickness of more than a hundred feet, beside the other strata exposed there.

The lower coal-measures disappear beneath the middle and upper measures to the southward and westward, but we may confidently expect to reach the continuation of those strata by sinking deep shafts in the western part of the State. It is not absolutely certain that coal will be found in those strata when they are so reached, but when a similar condition of the strata are found to exist in densely populated countries, capitalists do not hesitate to invest their money in such enterprises, and the time will doubtless come when it will be done in Western Iowa.

However, since coal-beds are liable to thin out as the distance increases from any given maximum point of development, and that all formations are expected to be thinnest at their borders, it will be less hazardous as well as less expensive to commence such enterprises in the valleys near the eastern or northern border of the upper or unproductive coal-measures, say in the counties of Decatur, Clarke, Madison and Adair; and if successful at those points it would encourage similar enterprises still farther and farther to the westward. Yet we do not know that the maximum development of the coal beds may not be to the westward of the Des Moines river region.

People throughout the State very often ask the following questions by letter and otherwise: What relative position does coal hold to limestone, sandstone or other strata? What is regarded as "coal - blossom," or a sure indication of the presence of coal beneath the surface? The term "coal-blossom" is not used by geologists, but those who use it appear to mean the presence of per oxide of iron, or simple iron-rust in the water of springy places, and impregnating the earth around where it issues. So far as that appearance is concerned it amounts only to this: The strata associated with the coal, and also the coal itself, almost invariably contain more or less iron, which being oxidized passes out with the water which finds its way over or through them. But while this is true, other strata which have no possible connection with coal, also at times are found to contain iron in a similar form, which of course presents similar appearances at the surface.

As to the relative position of coal-beds with other strata, the rule is that a bed of clay, commonly called fire - clay, underlies a bed of coal, while a bed of shale, often called soapstone by the people, almost always overlies it. Sometimes, however, sandstone, or even limestone, has been found to rest upon, or to underlie, a bed of coal.

In the following pages, a limestone will be referred to which underlies all the

coal-measure strata, and beneath which it is useless to search for coal. This is the sub-carboniferous limestone, and is usually found to belong to that division which geologists have named the St. Louis limestone, because the same formation is well exposed near that city. It is by no means meant that limestone is never found above the coal, for the upper coal- measures are largely composed of limestone, and the general appearance of the rock of these two limestone formations is often almost identical, yet when one is acquainted with the fossils which characterize each, he will know without hesitation whether a given exposure belongs above or below the productive coal measures. Since the lower coal measures contain almost no limestone, any such rock found in the region occupied by that formation, as for instance along the Des Moines River, may almost certainly be referred to the sub-carboniferous formation; because the streams, in cutting out their own valleys, have bared the limestone after they had cut clear through the coal - measure strata. From the fact that this limestone is so often exposed near the streams which run through the region occupied by the lower coal measures, it is evident that no very deep mining will be requisite in that region, to reach all the coal that exists there, because all the coal lying above the limestone will usually be obtained by drifting into the edges of the beds which crop out at the sides of the valleys. Strictly speaking, there are no indications that absolutely prove the presence of coal beneath any given spot, but since coal beds are known to have very considerable extent beneath the surface, and to lie in nearly a uni. form plane, one may often make very satisfactory calculations as to its presence or absence. Thus, if a bed of coal is found cropping out in one side of a valley, one may expect to find the same bed cropping out of the other side of it; and if a similar exposure should be found in another valley a few miles distant, it is reasonable to suppose, in our coal-field at least, that the two exposures are parts of the same bed, and that it is continuous beneath the whole intermediate space, and further, that it may be reached at an estimated depth by sinking a shaft down to it from the high land between the two valleys. Beside this, it will be seen from what has been said that the geologist is able to draw a line, at least approximately, and say "beyond this line it is useless to explore for coal, while within it there is a reasonable prospect of finding coal."

Mining in our State will always be mainly confined to coal, as will be seen by reference to descriptions of the extent and character of the coal-field on the following pages. Next in importance comes the lead mines of Dubuque, which although very important, occupy a very limited area. The present prospect that other mineral substances will be found in paying quantities is not encouraging. Occasional masses and smail deposits of iron ore are often found, but none have yet been discovered that will warrant the erection of a furnace, and even if it were so, as soon as our northern and southern railroads are built, iron from the Iron Mountain region in Missouri would doubtless be brought here more cheaply than we could manufacture it from our own scanty ores.

AUTHORSHIP.

The articles on "Surface Geology of the Middle Region of Western Iowa,'

"The Geology of the Raccoon River Region," and the report which bears his name, are the work of the Assistant. The articles on "The Fuel of lowa," "Waters of Iowa," "Rocks of Iowa," "Valuation, composition, and proximate analysis of coals," "Mineralogy of Iowa," together with the reports which bear his name are the work of the Chemist. All the remaining matter is by the State Geologist. As usual in such cases, each author is to be considered as responsible for his own work.

SURVEY OF COUNTIES.

BLACK HAWK AND BREMER COUNTIES.

HYDRAULIC CEMENT.

In April of 1866, while making the examinations referred to .in the preceding report, a communication was sent to the Waterloo Courier, which contained the following paragraphs:

Throughout the northeastern portion of the State, the prevailing character of nearly all the formations is magnesian limestone, which character extends to the Devonian rocks of your county, although the same formation in Johnson county, and elsewhere in the State, is but very slightly magnesian. It is a well known fact that rocks quite various in their chemical composition will nevertheless make, when properly prepared, excellent hydraulic cement, and among the rocks from which this cement is prepared are the very kinds of magnesian limestones which are found in the northeastern portion of the State. Those of your own vicinity are as well worthy of a trial for this result as any known to me in the Mississippi Valley, and specimens were collected from various quarries in your region with a view to having them analyzed and tested. It must be remarked, however, that the only reliable tests are those which may be made at an ordinary lime kiln, by properly burning the rock, and afterward grinding it very fine, and then by practical trial of the substance produced. When it has been determined that any rock will make the cement desired, the necessary skill can be readily acquired by any one for producing it in any quantity.

"It is the intention of Mr. McIsaac, of Waterloo, to make some practical tests of the magnesian limestones of your vicinity to which I have referred."

Within the last two months, Mr. McIsaac informed me that although