DAVENPORT PAST AND PRESENT

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CHAPTER XXIV.

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GEOLOGICAL SKETCH OF THE CITY OF DAVENPORT.

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The external features, and internal resources of any givendistrict of country, are intimately connected with its future history.  Fromits external features we gather in the main a knowledge of those habitablequalities which render it more or less desirable for civilized abodes.  Inits geographical position we learn the commercial advantages which attach to itslocation, as being accessibel or more remote, from business centres.

From a knowledge of its internal resources, we obtain theclearest insight to its productive capacities, determining in great measure theextent and character of its future population.

Hence it is that an accurate geological view of any district,affording information, both as regards external features and interanl resources,is important and useful as a key to its future history.

The Geological substratum upon which the city of Davenport islocated, is a white or light gray limestone, characterized by its fossils tobelong to the Hamilton group of Devonian Rocks.  This limestone crops cutalong the river banks, of the upper portion of the city.  It presents, nearEast Davenport, perpendicular cliffs, varying in height from 15 to 25 feet abovelow water mark; thence occupying the bed of the Mississippi river, it forms thelowest chain in the course of the Rock Island rapids, re-appearing again,similar in character, on Rock Island proper, and the corresponding left bank ofthe Mississippi.  The shores of both banks of the river are here strewnwith water-worn pebbles of this white limestone, variously mixed with smallerfragments of transported igneous rocks, including agates, cornelians, andnumerous froms of porphyry.

This bed of limestone underlies the whole city of Davenport,appearing on or near the surface at its south-eastern border, extending fromEast Davenport to Perry street.  Thence to the western limits of the cityit is more deeply covered under alluvial deposites.  This rock, togetherwith its alluvial covering, forms a gentle ascending slope from the river bankto the irregular line of bluff hills, which here bound the valley of theMississippi.  Where this rock is largely developed in steep mural faces, asadjoining and just below East Davenport, the bluffs approach near the riverbank, leaving little or no space for bottom levels.  This gives a somewhatrugged character to this locality.  In following the western course of theriver the limestone dips lower beneath  the surface, and the bluffs recede,thus giving greater width to the valley portion of the city towards its westernborder.

The bluff formation, attaining an average elevation of 150 feetabove the river level, presents on its outer edge abrupt slopes and roundedcrests, commanding extensive views of the course of the river above and below. Extending back from the river, this formation is cut up with deeplytrenched valleys, variously branched and thence emerging on the upland prairiebeyond.

These several features collectively, combine a pleasing varietyof external scenery, and offer grading facilities easy of application, and wellsuited for the purposes of drainage.

DEVONIAN LIMESTONE FORMATION.

Referring more particularly to the special characters of theformations above alluded to, we notice the underlying limestone strata to becomposed of a series of distinct beds, varying considerably in structure andcomposition.

First of these in a descdending order is an irregular shaly bed,containing the geater part of the fossils which serve to characterize thisfomation.  These strata are more largely developed to the south and west,being the common surface rock on both sides of the Mississippi, come eight orten miles below the city, at and adjoinging the town of New Buffalo.  Inthis latter locality the rocks are replete with fossils easily procured, and infine state of preservation.  Within the limits of the city this bed isexposed at only one locality, formerly known as LeClaire quarry, now foot ofFarnam street.  The rock here crops out just at the foot of the bluff, atan elevation of about forty feet above the river level.

To this fossiliferous bed succeeds the more common surfaceexposure, consisting of a white or light colored rock of slatey texture,weathering on exposure into thin irregular fragments.  This character ofrock shows a variable thinkness of from five to twenty feet, and is wellexhibited at the lower point of Rock Island, forming the greater part of theexposed rocky cliff on which old Fort Armstrong was built.

To this slatey rock succeeds a more compact bed, mostly massiveand heavy bedded.  Its texture varies from that of a close irregularbreccia of light color, and exceedingly brittle to loose strata of blueargilaceous rock, readily disintegrating on exposure to the atmosphere.  Intermediateto these we generally notice several seams of a more earthy gray rock occurringin even beds, and frequently containing masses of fibrous gypsum.  Theselatter seams furnish the best quality of building rock in this vicinity, beingin fact the only rock suitable for dressing under the hammer.  This seam isof very variabel thickness, being in some places entirely wanting, while inadjoing localities it attains a thickness of several feet.  The main bulkof the limestone quarries, being made up of the heavy bedded and irregularseamed rock, is only suitable for foundations or rough ashlar work.

Aside from building purposes this limestone contains no mineralsof any economical value, occasional spangles of sulphurate of zinc or moderatesized crystals of calcareous spar being the only minerals worthy of note.  Theslatey surface layers are employed for conversion into quick-lime, but theproduct is of rather indifferent quality.

One peculiarity of this limestone formation deserves more than apassing notice, both from its singularity and also its connexion with thesubterranean distribution of water.  This peculiarity consists in thefrequent occurence of fissures filled in with clay, evidently infiltrated fromabove.  These fissures or clay seams may be frequently noticed in theperpendicular face of quarries, here they are seen interrupting the regularseries of rock strata with masses of grayish, very adhesive clay.  Theseseams vary in width from a few inches to several feet, and are frequentlybottle-shaped, narrowing above and bulging out below.  Prof. Hall, StateGeologist, is inclined to the opinion that this clay is cotemporaneous with theunderlying fire-clay of adjoining coal measures, and that these fissures werefilled up at the same period that coal was in process of formation.  Theseclay seams are frequently met with in digging wells or deep cellar foundations,in which situations they are often accompanied with living springs of water. From such sources are evidently derived the supplies of water fromartesian borings, which have been made with partial success in various parts ofthe city.

BLUFF FORMATION.

The bluff formation constitutes a well marked step in the seriesof quarternary deposites, succeeding the drifter boulder era, and anterior tothe recent surface alluvium.  This formation, generally of considerablethickness, corresponding to the height of the bluff hills, forms the substratumof the upland prairies.  It is composed of a great variety of earthymaterials, including finely pulverulent marls, beds of coarse sand and gravel,aggregations resembling hard-pan or pudding-stone, overlaid by a variabel layerof yellow clay, and gradually blending with the present surface soil.  Theseseveral features indicate this formation as resulting from the deposition ofextensive fresh water lakes, having variabel currents and mostly shallow waters. Not unfrequently well excavations bring to view a buried soil of richvegetable mould now covered up by twenty feet or more of lacustrine deposites,containing fresh water shells.  This earlier surface soil supported a rankarborescent vegetation, and is proved by buried remains, to have been theroaming places of the now extinct tribes of the gigantic Mastodon and NorthernElephant.  The upper clay in the bluff series, is everywhere extensivelyused for the manufacture of brick.

A fine sectional view of the general features of this bluffformation may be seen in the cutting along the west side of Harrison street,opposite Sixth street.

COAL.

It would be interesting, did space allow to present some facts,in regard to the supply of coal in this district, but this must be left foranother occasion.  It will be sufficient here to state, that the onlyreliable supply of coal for this section of country, is to be obtained from theRock River coal basin.  This has been recently opened to market by twoRailroads, and is successfully worked by three distinct mining companies.  Thepresent facilities are sufficient to meet the local demand, and the source ofsupply is ample for all future wants.