TABLE XIV.-KEY TO THE MINERALS OF MONROE COUNTY.

Coherent,
in lumps

A cid
action on
solid
mineral is

Action is increased by powdering the mineral.

Light. Irregular brownish lumps of a loose, open structure.

will not burn.

Moderately heavy, imperfect cleavage, six-sided prisms.

Granular or in isolated cubes or octahedra, golden yellow color, slightly tarnished.

Flattened concretions, radially fibrous structure, silvery yellow, tarnished.

Very heavy. Rusty irregular lumps, generally loose in the soil.

Marcasité.

Does not effervesce with dilute acid.

Not scratched with finger

Scratched with finger

nail.

Effervesces with dilute

acid.

CHAPTER X.

FOSSILS OF MONROE COUNTY.

§ 1. General nature of fossils.

Embedded in the rocks which have been described, and imprinted upon the surfaces of the strata are the remains of former plant and animal life. From their resemblance to forms still in existence we know that they lived in the waters of an ocean charged with salt. From a study of the deposits which have been superposed, here and elsewhere, upon these strata we are forced to the conclusion that this ocean existed in Michigan so long ago that the time can be estimated only in millions of years. Nowhere in the county. or adjoining regions, have there been found embedded in the rocks. any trace of forms higher than the lowest order of fishes. When we consider the advance which has been made biologically since that time we are still better able to grant to these fossils the respect which is accorded great age. When such objects first began to attract attention during the 15th and 16th centuries they were regarded as "sports of nature," as though produced through Nature's effort at self amusement. By some writers they were thought to be due to the "influence of the stars," and by others to have resulted from the fermentation of certain fatty matter in the earth through the agency of heat. Early in the 16th century Fracastoro and Leonardo da Vinci declared that they represented the remains of organisms which had once lived where they are now found, a view that was savagely combatted for 200 years. As this simple truth finally prevailed over prejudice and superstition the question arose as to the time of their destruction, it being assumed that their death was simultaneous and produced by some widespread natural catastrophe. For a long time and by many the Noachian deluge was regarded as the cause of this universal destruction of life, of marine forms as well as those which lived upon the land. During the 18th century the discussions covered the method of extinction and entombment, the formation and displacement of the

rocks containing them, the succession of forms and their relation to living species. By the middle of the century it had been shown by Marsilli, Spada and Schiavo that fossils are not scattered at random through the rocks, but that there is an orderly succession. By the close of the century this truth had borne fruit. William Smith, an English engineer, suggested their use in recognizing and correlating strata, since beds of the same age should contain the same assemblage of fossils. During the century just closing thousands of forms have been described and figured, their groupings in the strata investigated, and the solution of the problems of development undertaken. From such study beds of economic importance may be located in the geological series and the physical and biological history of the earth deciphered.

By Lyell a fossil was defined as any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes. The cast or mould of any organism whatever, the imprint of a leaf or the foot print of an animal would thus come within the definition. So also would the body of a sheep buried by yesterday's land slide, but probably would not have been regarded as a fossil by Lyell himself. It is now generally understood that the form, in order to be considered a true fossil, must have lived during a previous geological epoch. § 2. Conditions of preservation.

With reference to their manner of preservation fossils may be separated into four classes. First, those in which the organic matter itself has been preserved as in the case of some of the plant remains to be described. Here the carbonaceous matter of the tissue is still in existence and makes up the fossil, although it has been mineralized. Rock oil represents such preserved organic matter which has become separated from the organisms of which it at one time constituted a part. In this same class should be placed bones, shells, corals, etc., which still retain the matter of which they were originally formed. In a second class of fossils this original matter has been replaced, particle by particle, by other mineral matter giving rise to a "petrifaction." The replacing mineral is most commonly calcite or silica, but may be pyrite, sulphur, iron oxide, etc. Very frequently, as in the case of silicified wood, the minute details of even the microscopic structure are most perfectly reproduced. In still a third class of fossils the organism itself, or some part of it, has left its impress upon a soft mass capable of retaining

the form. After hardening there is thus produced a mould of the original object. Such moulds of shells are quite common in the dolomites and occasionally a coral, or crustacean, is similarly preserved. Imprints of leaves, burrows and channels, footprints, etc., belong to this class. Finally, upon the withdrawal of the organism, or its removal by decay or solution, soft mineral matter may settle into the mould and form a cast. The most common form of cast is that of the entire exterior, but internal casts of shells are also not uncommon in the county. These are produced by the fine sedimentary material working its way into the internal cavity and there hardening. When the shell is dissolved away by percolating water a space is left between the mould and the internal cast. This occasionally becomes filled with sediment, sometimes with crystalline material and a cast of the shell itself is formed, which very much resembles a true petrifaction, but differs from it in not showing any of the real shell structure.

§ 3. Fossils of the Monroe beds.

The beds of dolomite are in general quite poor in fossils, but at certain horizons and in certain localities moulds and casts of brachiopods and gasteropods are abundant, crinoids, corals and bryozoan remains are found occasionally and lamellibranchs rarely. Worms, cephalopods and crustaceans are the only other animal groups that are known to be represented in this series of beds. There is nothing here to indicate that fish had yet come into exist ence, not a tooth, scale, plate or spine having yet been found. In nearly all cases the shell substance has been dissolved and nothing has been deposited in its place, so that the identifications have to be made upon the moulds, imprints, external and internal casts. A large number of such fragments were collected which are unidentifiable, either because of their state of preservation or because they represent species not yet described. The writer feels reasonably certain of the list below enumerated. A study was made of the fossil contents of the Monroe beds by Rominger and reported upon in 1876.* From these same beds in western Ohio, Whitfield identified and described a number of forms in a paper read before the New York Academy of Science in 1890.†

*Michigan Geological Survey, Vol. III, Pt. I, pp. 31 to 34.

Description of Fossils from the Paleozoic Rocks of Ohio. Ann. N. Y. Acad. of Science, 1890; pp. 505 to 622. Also Geol. Surv. of Ohio. Vol. VII. pp. 407 to 493. Plate V accompanying this paper is reproduced as plate XVII of this report through the kindness of Prof. Whitfied and the Ohio Survey, although it contains some forms not yet discovered in the county.

SPIRIFER VANUXEMI, Hall. Palæontology of New York, Vol.
III, 1859; p. 198, plate VIII, figs. 17 to 23.

Orthis plicata, Vanuxem. Geol. Rep. Third District of New
York, 1842; p. 112, fig. 1.

A few specimens of this species were found in the beds about Monroe, but they are not common. Adult forms of the type figured on plate XVII, figures 4 and 5, occur and also immature specimens similar to those drawn and described by Hall.

MERISTELLA LEVIS, Vanuxem, sp.

Atrypa lævis, Vanuxem. Geol. Rep. Third District of New York, 1842; p. 120, fig. 2.

Merista lavis, Hall. Palæontology of New York, Vol. III, 1859;
pp. 247-8, plate XXXIX, figs. 1 to 4.

Meristella lævis, Hall. Thirteenth Ann. Rep. of Regents on the
N. Y. State Cabinet, 1860; p. 75.

Cf. Whitfieldella lævis?, Grabau. Bull. Geol. Soc., Am. 1900, Vol.
XI, p. 369.

This is the most common and most widely distributed fossil in the county, being seen at nearly all the localities where the beds of dolomite are exposed. Its general form is shown in figures 6 and 7 of plate XVII.

MERISTELLA BELLA, Hall. Thirteenth Ann. Rep. of the Regents on the N. Y. State Cabinet, 1860; p. 75.

Merista bella, Hall. Palæontology of New York, Vol. III, 1859; p. 248, plate XL, fig. 1, a-p.

This form, which much resembles the preceding and is found in association with it, is figured on plate XVII, figures 8, 9, and 10. The general outline is seen to differ from Meristella lævis, and each valve possesses a sinus the meeting of which give a more emarginate character to the front.

NUCLEOSPIRA ROTUNDATA, Whitfield. Ann. N. Y. Academy of Science, 1882; p. 194.

Se plate XVII, figures 11 to 14. The best preserved specimens of this species were collected from the beds of the Woolmith quarry where they are associated with Meristella.