The year 1863 was very cold, not only in Iowa, but throughout the country, and there was frost in every month of the year, but it only once or twice during thirty years seriously injured the corn crop. When the spring is late the fall is generally lengthened, so that the crop has time to mature. The mean time for late spring frosts is May 4th; that of early fall frost is September 24th. The latest frost in the spring during thirty-one years, from 1839 to 1869, inclusive, was May 26th, 1847; and the earliest, August 29th,

1863.

Prairies.-The character of surface understood by the term prairie, is not a feature peculiar to Iowa, but is a characteristic of the greater portion of the Northwest. Dr. C. A. White, late State Geologist of Iowa, in his report says:

"By the word prairie we mean any considerable surface that is free from forest trees and shrubbery, and which is covered more or less thickly with grass and annual plants. This is also the popular understanding of the term. It is estimated that about seven-eighths of the surface of Iowa is prairie, or was so when the State was first settled. They are not confined to the level surface, but are sometimes even quite hilly and broken; and it has just been shown that they are not confined to any particular variety of soil, for they prevail equally upon Alluvial, Drift, and Lacustral soils. Indeed, we sometimes find a single prairie whose surface includes all these varieties, portions of which may be respectively sandy, gravelly, clayey or loamy. Neither are they confined to the region of, nor does their character seem at all dependent upon, the formations which underlie them, for within the State of Iowa they rest upon all formations, from those of Azoic to those of Cretaceous age inclusive, which embraces almost all kinds of rocks, such as quartzites, friable sandstone, magnesian limestone, common limestone, impure chalk, clay, clayey and sandy shales, etc. Southwestern Minnesota is almost one continuous prairie upon the drift which rests directly upon, not only the hard Sioux quartzite, but also directly upon the granite.

"Thus, whatever the origin of the prairies might have been, we have the positive assurance that their present existence in Iowa and immediate vicinity is not due to the influence of climate, the character or composition of the soil, nor to the character of any of the underlying formations. It now remains to say without the least hesitation, that the real cause of the present existence of prairies in Iowa, is the prevalence of the annual fires. If these had been prevented fifty years ago Iowa would now be a timbered instead of a prairie State.

"Then arises questions like the following, not easily answered, and for which no answers are at present proposed:

"When was fire first introduced upon the prairies, and how? Could any but human agency have introduced annual fires upon them? If they could have been introduced only by the agency of man why did the forests not occupy the prairies before man came to introduce his fires, since we see their great tendency to encroach upon the prairies as soon as the fires are made to cease? The prairies, doubtless, existed as such almost immediately after the close of the Glacial epoch. Did man then exist and possess the use of fire that he might have annually burnt the prairies of so large a part of the continent, and thus have constantly prevented the encroachments of the forests? It may be that these questions will never be satisfactorily answered; but nothing is more evident than that the forests would soon occupy a very large proportion of the prairie region of North America if the prai

rie fires were made to cease, and no artificial efforts were made to prevent their growth and encroachment."

Soils.-Dr. White has separated the soils of Iowa into three general divisions, viz: the Drift, Bluff, and Alluvial. The drift soil occupies the greater portion of the State, the bluff next, and the alluvial the least. The drift is derived primarily from the disintegration of rocks, to a considerable extent perhaps from those of Minnesota, which were subject to violent glacial action during the glacial epoch. This soil is excellent, and is generally free from coarse drift materials, especially near the surface.

The bluff soil occupies an area estimated at about five thousand square miles, in the western part of the State. It has many peculiar and marked characteristics, and is believed to be lacustral in its origin. In some places the deposit is as great as two hundred feet in thickness, all portions of it being equal in fertility. If this soil be taken from its lowest depth, say two hundred feet below the surface, vegetation germinates and thrives as readily in it as in the surface deposit. It is of a slightly yellowish ash color, except when mixed with decaying vegetation. It is composed mainly of silica, but the silicious matter is so finely pulverized that the naked eye is unable to perceive anything like sand in its composition. The bluffs along the Missouri river, in the western part of the State, are composed of this material.

The alluvial soils are the "bottom" lands along the rivers and smaller streams. They are the washings of other soils mixed with decayed vegetable matter. They vary somewhat in character and fertility, but the best of them are regarded as the most fertile soils in the State.

As to the localities occupied by each of these different soils, it may be stated that the drift forms the soil of all the higher plains and woodlands of the State, except a belt along the western border, which is occupied by the bluff soil, or bluff deposit, as it is generally called. The alluvial occupies the low lands, both prairie and timber, along the streams. It may be remarked that the alluvial soil composing the broad belt of "bottom" along the Missouri, partakes largely of the bluff soil, owing to continued washings from the high lands or bluffs adjacent.

GEOLOGY OF IOWA.

Classification of Rocks-Azoic System-Huronian Group-Lower Silurian System-Primordial Group-Trenton Group-Cincinnati Group-Upper Silurian System-Niagara GroupDevonian System-Hamilton Group-Carboniferous System-Sub-Carboniferous GroupKinderhook Beds—Burlington Limestone-Keokuk Limestone St. Louis LimestoneCoal-Measure Group-Cretaceous System-Nishnabotany Sandstone-Woodbury Sandstones and Shales-Inoceramus Beds.

IN January, 1855, the General Assembly passed an act to provide for a geological survey of the State. Under authority given by this act, Prof. James Hall, of New York, was appointed State Geologist, and Prof. J. D. Whitney, of Massachusetts, State Chemist. During the years 1855, 1856, and 1857, the work progressed, but was confined chiefly to the eastern counties. A large volume was published in two parts, giving in detail the results of the survey up to the close of the season of 1857, when the work was discontinued. In 1866 it was resumed under an act of the General Assembly passed in March of that year, and Dr. Charles A. White, of Iowa City, was appointed State Geologist. He continued the work, and in December, 1869,

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 cracks. 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 firm 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 uses 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. It occupies 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

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, socalled 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.