334 IOWA lower formations continue to appear in succes- sion, and stretch in long parallel belts N. W. and S. E. The course of the rivers, as they descend from the N. border of the state to the Mississippi, is along the range of these forma- tions; and it is remarkable how each river keeps almost exclusively along the same geolo- gical belt for nearly 200 in. The successive belts are thus designated by Prof. Hall, ad- vancing N. E. in the descending series from the coal measures and the underlying carbonif- erous limestone: the Hamilton and Ohemung groups of the Devonian series, the Leclaire and Niagara limestones of the upper Silurian, the Hudson river shales, Galena limestone, Tren- ton limestone, St. Peter's sandstone, and Pots- dam sandstone of the lower Silurian. The last occupy the N. E. corner of the state. The coal measures are regarded as the most permanent source of mineral wealth, though the lead mines in the Galena limestone have attracted the ear- liest and greatest attention, and have so far been the most important in the value of their productions. Their aggregate thickness in the S. part of the state is less than 500 ft., and in this are found several workable beds of bi- tuminous coal, one of which is sometimes 7 ft. thick. The formation thins out as it spreads over the carboniferous limestone, and in this are found several outlying shallow basins of small extent. They are also met with beyond the limits' of this rock, scattered in the depres- sions of the Devonian, and even the Silurian series, and resting unconformably upon the up- turned edges of these lower formations. Along the Mississippi river, between Davenport and Muscatine, a deposit of this character stretches 20 m. up and down the river, and not more than 3 m. back from it. This is the extension on the Iowa side of the Illinois coal field, the Mississippi river having completely separated this marginal portion from the main body. The thickness of this isolated group of coal measures is not more than about 70 ft. It con- tains one coal bed which is identified as the lowest workable coal of the series; it is of no great importance, being only about 2 ft. thick. A few yards beneath it is a bed of carbona- ceous slate, which sometimes presents a seam of cannel coal a foot thick. The coal field of the state embraces an area of about 20,000 sq. m. ; the coal is bituminous and of excellent quality. In the N. part of the state are extensive beds of superior peat from 4 ft. to 10 ft. in depth. The lead mines are in the belt occupied by the Galena limestone. This tract reaches the Mis- sissippi river at Dubuque, and lies along the valley of Turkey river toward the N. W. ; but the only mines that have been worked in it are near the Mississippi. The ore is chiefly found in vertical crevices which are traced in E. and W. lines with remarkable regularity. They are congregated in great number in the immediate vicinity of Dubuque, and from the report of the state geologists it appears that no district in the Mississippi valley has produced so large an amount of ore for its ex- tent as this tract of 12 or 15 sq. m. From 4,000,000 to 6,000,000 Ibs. of ore have been smelted annually at the Dubuque mines, yield- ing about 70 per cent, of lead. The crevices in the limestone are frequently found to ex- pand into what are called openings and large caves several hundred feet long. The walls of these are incrusted with the sulphuret of lead, of which a single cave sometimes furnishes sev- eral million pounds. The depth of the mines is limited to the thickness of the Galena lime- stone, which seldom reaches 200 ft. In the blue limestone, which underlies it, the crevices either close up or are unproductive. The yield of ore is very irregular, and the same mines rarely continue to be worked for even a few years. (See LEAD.) Iowa has also small de- posits of iron ore, and there are many other minerals of considerable value. A deposit of gypsum of remarkable fineness and purity exists near Fort Dodge ; it is confined to an area of about 6 by 3 m. on both sides of the Des Moines river, and is from 25 to 30 ft. thick. Plaster of Paris of superior quality has been manu- factured from it. Building stone of the best description, various clays, &c., sufficient for all present or prospective requirements, are found. The soils of Iowa are generally excellent, and no state of the Union has a smaller amount of inferior land. The valleys of the Red Cedar, Iowa, and Des Moines, as high as lat. 42 30', present a body of arable land which, taken as a whole, for richness in organic elements, for amount of saline matter, and due admixture of earthy silicates, affords a combination that belongs only to the most fertile upland plains. North of this, the best agricultural region of the state, the lands are inferior, but still not unprofitable, and the lower grounds are either wet and marshy or filled with numerous ponds, and entirely destitute of timber. The climate of Iowa is moderate, and highly favorable for agricultural operations. As a general rule the peach blossoms in mid April, and wheat ripens early in August. The winters, however, are severe from the prevalence of N. and N. W. winds, which sweep over the level prairies with- out obstruction ; but they are very equable and healthful. In summer the winds are from the west and south, and, being constant, greatly relieve the heats of that season. The mean temperature of the year is about 48 F. ; spring, 47^ ; summer, 70-J-" ; autumn, 45 ; win- ter, 23. The highest temperature observed for 30 years was 99, Aug. 31, 1854; the lowest 30, Jan. 18, 1857. The temperature is sel- dom lower than 10, or higher than 90. The mean annual amount of rainfall for 30 years was 44-27 in. ; least, 23-35 in. ; greatest, 74-49 in. Iowa is classed among the most healthy coun- tries of the world, a fact to be attributed to the excellent drainage furnished by its rolling surface. The exceptions to this condition are very rare. The natural growths of Iowa are similar to those of the whole middle zone of the
