We are indebted to His Honor, Mayor Amos V. Eaton, of Anamosa, for the following in reference to the geology of the county. Mayor Eaton is quite an industrious student of the natural history of the county and State, and has succeeded, through his own efforts, in collecting and arranging a valuable cabinet, comprising all the important specimens of this portion of the State, and many valuable specimens from other portions of the United States and the Canadas:
The geological formation of Jones County is almost wholly within the Upper Silurian age and the Niagara period. The Devonian laps over upon a small portion of Greenfield Township. The Upper Silurian exposure in the State is something in the shape of a pyramid, with its base of about fifty miles in width on the Mississippi River, and includes the county of Clinton and a large portion of the counties of Jackson and Scott, tapering to a width of not more than four or five miles on the Minnesota line, in Howard County, with a length of 160 miles, extending northwest and southeast. The thickness of the formation is set down at 350 feet.
In the early surveys of the State, the Niagara period was divided into the Niagara and the Le Claire epochs and formations; but subsequent surveys and examinations determined the fact that it properly all belongs to the Niagara epoch. The rock of the formation is a magnesia limestone, and, in an economic view, one of the most important in the State. Prof. White makes the statement that it affords the best nd greatest amount of quarry rock of any formation in the State of Iowa. Wherever this rock is exposed in Jones County, it furnishes a great abundance of material for the common uses of the inhabitants. The exposure of this stone near Anamosa is of such wonderful regularity in the stratification and such uniformity of texture that the stone can be wrought into any desired shape or size with little expense. Some of the stone come from their beds as smooth and even as though they had been run through a planing-machine, not requiring the tough of the chisel. Another very fortunate thing, there are no intervening strata of clay or other material to impede the labors of the quarrymen. This stone weighs about one hundred and fifty pounds per cubic foot, getting a little heavier as you go down the quarry. The strength and durability of the stone has been tested under all manner of conditions for years, and it is all that could be reasonably expected of a limestone. Exposure to the atmosphere improves it, making it harder. The analysis of the stone, as given by Prof. White in his report, is as follows:
One noteworthy fact, geologically, in relation to these special quarries, is that there is hardly any fossil remains to be found in them, while they are quite abundant in other places not far away. A few traces of coral are found, and a formation that has not been determined as yet, although examined by some eminent geologists. They are about one-half inch long and as large as a knitting-needle; and, as far as can be seen, they are exactly alike and often occur in innumerable numbers, in a single slab of stone. The impression is more often, seen, however, and indeed it is not certain whether they are fossils at all or not. A flint nodule of very beautiful structure is found, being many times found in layers of strata, and furnishing very unique cabinet specimens. Pockets containing quartz and lime crystals frequently occur that are elegant in appearance.
The surface soil of Jones County is composed of what is termed drift and alluvial soils, the former largely predominating, as the flood-plane or bottomland of the rivers is not great. Prof. White estimates that 95 per cent of the land in Iowa is tillable, and Jones County is quite equal to the average. If Prof. White's estimate of the State is not exaggerated, there is probably no other area of territory of the extent (55,000 square miles) in the world, that can furnish as good and as large a per cent of tillable land as the State of Iowa.
The soil of the county possesses the ingredients and depth to make it inexhaustible with fair dealing, and insuring its inhabitants an agricultural wealth forever.
There is enough of good brick clay in the county to furnish its inhabitants with brick for all time to come. Any there is sand enough along the streams, that has been sifted from the soil by the action of water, to furnish the requisite quantity for building purposes, and an endless amount of stone that makes good (quick) lime.
The county is almost destitute of minerals as far as known; a few isolated specimens of iron ore have been found, and traces of iron in the rock material are sometimes seen. As the Silurian age is below the coal formations, it would be useless to look for coal in the county.
The paleontology, or fossil remains, of Jones County has been almost wholly neglected or overlooked by the State geologists, in their surveys and reports, and very little has been written upon the subject; and while we cannot claim as much of interest in this direction as many counties of the State, still there is abundance of material to interest the geologist. The Silurian formation is one of extreme age. Some geologists of authority have put it down as having taken millions of years in its formation. And as it was the first in which life began to show itself on the globe-life in the simplest form-it is called the age of mollusks, because they are so predominate. The word mollusk means soft, and the animals are composed of a soft, fleshy bag, containing a very simple digestive apparatus. Many of them are without eyes, and are generally covered with a shell as a means of protection. The clam, snail and oyster are familiar examples of this class, now living; but many of the fossils now found are the remains of species now extinct.
However, the fossil remains of this county are composed quite as largely of the class called radiates, which are quite as simple in structure, and might be called the stepping-stone from the vegetable to the animal kingdom. The corals and crinoids are examples of this class.
Much of the rock exposure in the county is nearly destitute of fossils, while in others they are very numerous. The following are the more common ones found in Jones County:
Several species of the favosites corals (honeycomb corals) are very numerous; two or three species of halysites (chain corals); a number of syringoropora (pine coral); cyathophylloid; stromatapora; chonophyllum (cup coral) and heliolites.
All of the above are found in one locality along the Maquoketa River, a few miles east of Monticello, in such quantities that wagon-loads may be gathered of those that lie loose on the surface of the bluffs.
Other species of corals are found in various parts of the county, that are more rare, and many that seem to be peculiar to this formation. Two species, at least, of pentamerons are occasionally found, but are much more numerous over the line in Linn County. Crinoid remains are very common in many places.
One locality near Anamosa, on the Wapsipinicon River, at Doan's Mill, the stone is entirely made up of them, but it is so rotten and fragmentary in character that complete specimens are obtained with difficulty. Enough of comparatively perfect crinoid heads have been found to identify several species. The stone is sufficiently made up of them to justly entitle it to the name of crinoid limestone.
Fossil shells are not numerous, but several species, both of the bivalve and univalve, have been found.
Trilobites are very rare in this county, although in some of the Silurian formations they are numerous; 500 species of this crustacean once existed, all of which are now extinct. (Dana.)
During this season, a point of rocks has been opened near the iron bridge across the Buffalo at Fisherville, where the trilobite s are quite numerous. The quarrymen inform me they found at least 100 in number, and that they only occur in one or two strata, as far as yet developed. Only three other ones have been found in the county, to my knowledge.
Several ammonites have been found, but they are also rare; 900 species of these animals once existed and are now extinct. (Dana.)
Several species of the orthoceras and also of the ormoceras are met with, although they are not often found complete.
Specimens of so-called iron-stone and agatized flint are often found, and, indeed, the flint formations of the county often take on a wonderful variety of forms and fantastic shapes. The variety called the jasper is frequently found. Specimens of what is termed forest rock are sometimes found in the quarries, and are thought by some to be fossil ferns. It is simply a precipitate of oxide of manganese. Fossils of the vegetable kingdom are not found to any extent whatever. A few pieces of petrified wood have been found along the streams, but they are evidently foreign, and brought here by the drift.
Much of that which is interesting to the student of natural history might be written in reference to the geology of Jones County, but the subject has not attracted sufficiently the attention of the inhabitants to warrant anything further in a county history. The geology of Jones County affords abundant opportunity to those of her citizens who may desire to gain a practical knowledge of a subject that has entirely revolutionized the thinking world during the past half-century.