saturated with sulphate of lime and destitute of life; its stratification and impurities being deposited at the same time as clayey impurities which had been held suspended in the same waters.

Physical Properties.—Much has already been said of the physical properties or character of this gypsum, but as it is so different in some respects from that of other deposits, there are yet other matters worthy of mention in connection with those. According to the results of a complete and exhaustive analysis by Prof. Emery, the ordinary gray gypsum contains only about eight per cent. of impurity; and it is possible that the average impurity for the whole deposit will not exceed that proportion, so uniform in quality is it from to top to bottom and from one end of the region to the other.

When it is remembered that plaster for agricultural purposes is sometimes prepared from gypsum that contains as much as thirty per cent. of impurity, it will be seen that ours is a very superior article for such purposes. The impurities are also of such a character that they do not in any way interfere with its value for use in the arts. Although the gypsum rock has a gray color, it becomes quite white by grinding, and still whiter by the calcining process necessary in the preparation of plaster of Paris. These tests have all been practically made in the rooms of the Geological Survey, and the quality of the plaster of Paris still further tested by actual use and experiment. No hesitation, therefore, is felt in stating that the Fort Dodge gypsum is of as good a quality as any in the country, even for the finest uses.

In view of the bounteousness of the primitive fertility of our Iowa soils, many persons forget that a time may come when Nature will refuse to respond so generously to our demand as she does now, without an adequate return. Such are apt to say that this vast deposit of gypsum is valueless to our commonwealth, except to the small extent that it may be used in the arts. This is undoubtedly a short-sighted view of the subject, for the time is even now rapidly passing away when a man may purchase a new farm for less money than he can re-fertilize and restore the partially wasted primitive fertility of the one he now occupies. There are farms even now in a large part of the older settled portions of the State that would be greatly benefited by the proper application of plaster, and such areas will continue to increase until it will be difficult to estimate the value of the deposit of gypsum at Fort Dodge. It should be remembered, also, that the inhabitants of an extent of country adjoining our State more than three times as great as its own area will find it more convenient to obtain their supplies from Fort Dodge than from any other

source.

For want of direct railroad communication between this region and other parts of the State, the only use yet made of the gypsum by the inhabitants is for the purposes of ordinary building stone. It is so compact that it is found to be comparatively unaffected by the frost, and its ordinary situation in walls of houses is such that it is protected from the dissolving action of water, which

can at most reach it only from occasional rains, and the effect of these is too slight to be perceived after the lapse of several years.

One of the citizens of Fort Dodge, Hon. John F. Duncombe, built a large, fine residence of it, in 1861, the walls of which appear as unaffected by exposure and as beautiful as they were when first erected. It has been so long and successfully used for building stone by the inhabitants that they now prefer it to the limestone of good quality, which also exists in the immediate vicinity. This preference is due to the cheapness of the gypsum, as compared with the stone. The cheapness of the former is largely due to the facility with which it is quarried and wrought. Several other houses have been constructed of it in Fort Dodge, including the depot building of the Dubuque & Sioux City Railroad. The company have also constructed a large culvert of the same material to span a creek near the town, limestone only being used for the lower courses, which come in contact with the water. It is a fine arch, each stone of gypsum being nicely hewn, and it will doubtless prove a very durable one. Many of the sidewalks in the town are made of the slabs or flags of gypsum which occur in some of the quarries in the form of thin layers. They are more durable than their softness would lead one to suppose. They also possess an advantage over stone in not becoming slippery when worn.

The method adopted in quarrying and dressing the blocks of gypsum is peculiar, and quite unlike that adopted in similar treatment of ordinary stone. Taking a stout auger-bit of an ordinary brace, such as is used by carpenters, and filing the cutting parts of it into a peculiar form, the quarryman bores his holes into the gypsum quarry for blasting, in the same manner and with as great facility as a carpenter would bore hard wood. The pieces being loosened by blasting, they are broken up with sledges into convenient sizes, or hewn into the desired shapes by means of hatchets or ordinary chopping axes, or cut by means of ordinary wood-saws. So little grit does the gypsum contain that these tools, made for working wood, are found to be better adapted for working the former substance than those tools are which are universally used for working stone.

MINOR DEPOSITS OF SULPHATE OF LIME.

Besides the great gypsum deposit of Fort Dodge, sulphate of lime in the various forms of fibrous gypsum, selenite, and small, amorphous masses, has also been discovered in various formations in different parts of the State, including the coal measure shales near Fort Dodge, where it exists in small quantities, quite independently of the great gypsum deposit there. The quantity of gypsum in these minor deposits is always too small to be of any practical value, and frequently minute. They usually occur in shales and shaly clays, associated with strata that contain more or less sulphuret of iron (iron pyrites). Gypsum has thus been detected in the coal measures, the St. Louis limestone, the cretaceous strata, and also in the lead caves of Dubuque. In most of these cases it is evidently the result of double decomposition of iron pyrites and car

bonate of lime, previously existing there; in which cases the gypsum is of course not an original deposit as the great one at Fort Dodge is supposed to be.

The existence of these comparatively minute quantities of gypsum in the shales of the coal measures and the subcarboniferous limestone which are exposed within the region of and occupy a stratigraphical position beneath the great gypsum deposits, suggests the possibility that the former may have originated as a precipitate from percolating waters, holding gypsum in solution which they had derived from that deposit in passing over or through it. Since, however, the same substance is found in similar small quantities and under similar conditions in regions where they could have had no possible connection with that deposit, it is believed that none of those mentioned have necessarily originated from it, not even those that are found in close proximity to it.

The gypsum found in the lead caves is usually in the form of efflorescent fibers, and is always in small quantity. In the lower coal-measure shale near Fort Dodge, a small mass was found in the form of an intercalated layer, which had a distinct fibrous structure, the fibers being perpendicular to the plane of the layer. The same mass had also distinct, horizontal planes of cleavage at right angles with the perpendicular fibers. Thus, being more or less transparent, the mass combined the characters of both fibrous gypsum and selenite. No anhydrous sulphate of lime (anhydrite) has been found in connection with the great gypsum deposit, nor elsewhere in Iowa, so far as yet known.

SULPHATE OF STRONTIA.

(Celestine.)

The only locality at which this interesting mineral has yet been found in Iowa, or, so far as is known, in the great valley of the Mississippi, is at Fort Dodge. It occurs there in very small quantity in both the shales of the lower coal measures and in the clays that overlie the gypsum deposit, and which are regarded as of the same age with it. The first is just below the city, near Rees' coal bank, and occurs as a layer intercalated among the coal measure shales, amounting in quantity to only a few hundred pounds' weight. The mineral is fibrous and crystalline, the fibers being perpendicular to the plane of the layer. Breaking also with more or less distinct horizontal planes of cleavage, it resembles, in physical character, the layer of fibro-crystalline gypsum before mentioned. Its color is light blue, is transparent and shows crystaline facets upon both the upper and under surfaces of the layer; those of the upper surface being smallest and most numerous. It breaks up readily into small masses along the lines of the perpendicular fibers or columns. The layer is probably not more than a rod in extent in any direction and about three inches in maximum thickness. Apparent lines of stratification occur in it, corresponding with those of the shales which imbed it.

The other deposit was still smaller in amount, and occurred as a mass of crystals imbedded in the clays that overlie the gypsum at Cummins' quarry in

the valley of Soldier Creek, upon the north side of the town. The mineral is in this case nearly colorless, and but for the form of the separate crystals would closely resemble masses of impure salt. The crystals are so closely aggregated that they enclose but little impurity in the mass, but in almost all cases their fundamental forms are obscured. This mineral has almost no real practical value, and its occurrence, as described, is interesting only as a mineralogical fact.

SULPHATE OF BARYTA.

(Barytes, Heavy Spar.)

This mineral has been found only in minute quantities in Iowa. It has been detected in the coal-measure shales of Decatur, Madison and Marion Counties, the Devonian limestone of Johnson and Bremer Counties and in the lead caves of Dubuque. In all these cases, it is in the form of crystals or small crystalline masses.

SULPHATE OF MAGNESIA.

(Epsomite.)

Epsomite, or native epsom salts, having been discovered near Burlington, we have thus recognized in Iowa all the sulphates of the alkaline earths of natural origin; all of them, except the sulphate of lime, being in very small quantity. Even if the sulphate of magnesia were produced in nature, in large quantities, it is so very soluble that it can accumulate only in such positions as afford it complete shelter from the rains or running water. The epsomite mentioned was found beneath the overhanging cliff of Burlington limestone, near Starr's mill, which are represented in the sketch upon another page, illustrating the subcarboniferous rocks. It occurs in the form of efflorescent encrustations upon the surface of stones and in similar small fragile masses among the fine debris that has fallen down beneath the overhanging cliff. The projection of the cliff over the perpendicular face of the strata beneath amounts to near twenty feet at the point where epsomite was found. Consequently the rains never reach far beneath it from any quarter. The rock upon which the epsomite accumulates is an impure limestone, containing also some carbonate of magnesia, together with a small proportion of iron pyrites in a finely divided condition. It is doubtless by double decomposition of these that the epsomite results. By experiments with this native salt in the office of the Survey, a fine article of epsom salts was produced, but the quantity that might be annually obtained there would amount to only a few pounds, and of course is of no practical value whatever, on account of its cheapness in the market.

CLIMATOLOGY.

No extended record of the climatology of Iowa has been made, yet much of great value may be learned from observations made at a single point. Prof. T. S. Parvin, of the State University, has recorded observations made from 1839 to the present time. Previous to 1860, these observations were made at Mus

catine. Since that date, they were made in Iowa City. The result is that the atmospheric conditions of the climate of Iowa are in the highest degree favorable to health.

The highest temperature here occurs in August, while July is the hottest month in the year by two degrees, and January the coldest by three degrees.

The mean temperature of April and October most nearly corresponds to the mean temperature of the year, as well as their seasons of Spring and Fall, while that of Summer and Winter is best represented in that of August and December.

The period of greatest heat ranges from June 22d to August 31st; the next mean time being July 27th. The lowest temperature extends from December 16th to February 15th, the average being January 20th-the range in each case being two full months.

The climate of Iowa embraces the range of that of New York, Pennsylvania, Ohio, Indiana and Illinois. The seasons are not characterized by the frequent and sudden changes so common in the latitudes further south. The temperature of the Winters is somewhat lower than States eastward, but of other seasons it is higher. The atmosphere is dry and invigorating. The surface of the State being free at all seasons of the year from stagnant water, with good breezes at nearly all seasons, the miasmatic and pulmonary diseases are unknown. Mortuary statistics show this to be one of the most healthful States in the Union, being one death to every ninety-four persons. The Spring, Summer and Fall months are delightful; indeed, the glory of Iowa is her Autumn, and nothing can transcend the splendor of her Indian Summer, which lasts for weeks, and finally blends, almost imperceptibly, into Winter.