southern outcrops of these beds. In the case of the Stewart boulder described, we have an apparently identical mass of limestone lying to the south of the outcrop supposed to have furnished the others. The position of the Corniferous, Hamilton and Marshall beds in eastern Michigan and western Ontario is such that a movement of the ice sheet S. 60° W. would account for the location of the most northern masses described as occurring in this portion of the state, with a maximum journey less than one-half that required to bring them from the northern outcrops. At Stony Point Winchell found glacial striations bearing S. 60° W. This same set occurs at Point aux Peaux, and strikes S. 65° W. at Brest, according to the earlier observations of Hubbard. The glacial striæ, however, found at Trenton and several localities in Monroe county show conclusively that the ice sheet itself actually moved northwestward upon the northern side of the Erie lobe (Chapter VI, § 7), so that the journey made by the rock masses may have been a still shorter one. Much more extensive masses of the same character occur in northern Illinois near Freeport and were described in 1897 by Hershey in a paper upon "The Eskers of the Kansan Epoch." These are referred to later by Leverett in his recent monograph on the Illinois Glacial Lobe under the heading "Transported rock ledges." Some of these masses. have not been so gently handled and consist in part of angular fragments of the limestone. Two of the largest taken together are stated to cover 100 acres and to attain a maximum thickness of 40 feet. According to Hershey they were pushed along in front of the ice and were never overridden by it.

Only in Berlin and eastern Frenchtown is there any bunching of this surface material sufficient to suggest a morainic deposit. From Stony Point northward the boulders are more abundant, especially upon the "ridge," where the covering of clay is light. In several places the ground is greatly encumbered with cobble-stones, especially upon claims 528, 529, 530 and 531 (N. Stony Creek). In Berlin township, S. W. 1, Sec. 33, the same cobbles occur in great abundance. It will be shown in another connection that these deposits mark the southern prolongation of a well defined, water-laid moraine, extending from the Huron River northward past Trenton, towards Detroit. (Chapter VI, § 8.)

*American Geologist, Vol. XIX, pp. 197-209, 237-253.

*The Illinois Glacial Lobe. Monograph XXXVIII, U. S. Geol. Surv., 1899. pp. 82-84.

$ 4. Post-glacial deposits.

Under the head of Quaternary belong not only the drift deposits described but others of an entirely different nature and history, which will be only briefly referred to here. Spread over nearly the entire till area of the county, is a layer of dark clay loam; very thin or absent in the western part, but increasing in thickness toward the east and southeast. Its absence over certain areas may be due to erosive agencies. It is quite free from pebbles, shows no distinct stratification, owes its dark color to organic matter and is, agriculturally, of very great importance to the county. No sharp line of demarcation exists between this superficial deposit and the underlying till, one seeming to grade into the other.

Passing northward from Sylvania, Ohio, to Oakville is a broad, irregular belt of surface sand, varying in breadth from two to eight miles. In many places the sand has been heaped up by wind action into mounds and ridges, the latter of which sometimes squirm across several adjoining sections. Near Temperance there branches off from this main belt a secondary one, extending northeastward to Carleton and beyond (See map, Plate VII). This is narrower and much more irregular, but otherwise similar. Transverse belts of similar sand almost completely connect these two belts in Raisinville and Exeter townships. From Milan village there extends southwestward across the township, through Dundee to the northwestern corner of Summerfield, a fairly continuous, but narrow strip of yellow sand, with occasional pebbly patches upon either side.

In addition to the clay and sand deposits of such wide extent, there are others, of a more local nature, to which attention may here be called. Along the rivers and smaller streams terraces of silt have been formed, of considerable breadth and thickness and consisting of clay, sand and organic matter. The surfaces of these terraces constitute the flood plains, in general, and receive slight additions with each inundation. Locally, where there have been lakes and marshes, or where these still exist, beds of marl, peat and bog-ore are found, or are now in process of formation. About some of the extensive springs, the waters of which are highly charged with lime carbonate, deposits of calcareous matter over leaves, twigs and moss, result in the formation of a "tufa."

B. Devonian Formations.-St. Clair (Genesee) Shales.

§ 5. General arrangement of Devonian formations.

If all the deposits which have just been described were removed and a general view could be had of the so called "bed rock," it would appear, approximately, as is shown upon the map (Plate I). This constitutes what is called a geological map, in which the belts of color represent certain groups of strata, which have a common structure, composition and history. Being formed within the limits of the same geological period certain distinctive names are assigned to these groups, or to their subdivisions. In Monroe County these strata all have a general northwesterly "dip," with their outcropping edges extending ("striking"), in a northeast and southwest direction. Obviously the strata which were latest formed are in the northwestern corner of the county and the next oldest, in succession, would be encountered as one passed southeastward, across the line of strike. This order of position will determine the order of description of the rock formations.

§ 6. Name.

The youngest member of this Monroe County series is the group of "St. Clair shales,' so named from their occurrence along St. Clair River. They constitute the lower portion of Winchell's "Huron Group," described in his report of 1861 (pp. 71-80). In the early Ohio reports the beds are referred to as the "Huron shale," "Black shale," "Black slate," and "Shale stratum." Newberry in his report of 1873† describes two beds of shale, the Erie and Cleveland, which immediately overlie the Huron. These three were united by Orton into a single stratum which he termed the "Ohio Shale." An upper and a lower division were recognized, for the latter of which he retained the name Huron shale, and it is this which forms the northwestern corner of Monroe County (See Map I). It is probably nearly equivalent to the Genesee shale of the New York series.

§ 7. General data.

Only about seven sections of the northwestern corner of Milan township are underlain by these shales, the main portion of the stratum lying in Lenawee and Washtenaw counties, and dipping at a low angle to the northwest. They are covered by 100 to 150 feet

*Geol. Sur. of Mich., Vol. V, 1895, pt. II, p. 21. Geol. Sur. of Ohio, Vol. I, pp. 152-167, 189-191. Geol. Sur. of Ohio, Vol. VII, 1893, pp. 21-26.

+

of drift and so are struck only in the relatively deep wells of this region. The actual elevation of the base of the stratum is about 560 feet above tide at Milan (18 feet below the level of Lake Erie), while the elevation rises to 610 feet where it leaves the county. The highest elevation noted is in the extreme northwestern corner of Section 6, where it is about 625 feet above tide. Winchell gives an outcrop of shale of his Huron group as occurring near Adrian, Lenawee County.* If such an outcrop occurs the shales cannot be the St. Clair since these were struck in the well of the Adrian Gas Company at a depth of 524 feet from the surface. Good exposures, however, are found northward about the shores of Lake Huron and along some of the small ravines leading back from the lake.

§ 8. Thickness.

In the Adrian well above referred to, the shales regarded by Lane as the St. Clair show a thickness of 221 feet. At Ann Arbor in the court-yard well, they appear to have a thickness of 275 feet, according to the same authority. In Ohio these shales show a development of about 300 feet and in western Ontario at Corunna, Lambton county, 213 feet. At Sarnia, according to Brumell, they are reduced to 80 feet. The base of the bed at Ann Arbor has an elevation of 134 feet above tide; at Milan 560 feet, giving a drop in the fourteen miles, in this direction, of 426 feet, or about 30.5 feet to the mile. § 9. Lithological character.

Where well exposed, as upon Sulphur Island, Thunder Bay, these shales are seen to be black, very fissile, hard, crisp and sharp; approaching slate, when unweathered. They possess a certain amount of elasticity, and snap and crackle under the feet, as one walks along the beach. They are finely laminated and very evenly bedded. When exposed to the weather they assume a gray, or rusty brown color, from the oxidation of the contained iron. Efflorescences of sulphates of iron, or alumina, form over the surface where it is somewhat protected from the rain. The color becomes darker towards the bottom of the series, due to the greater amount of carbonaceous matter derived from vegetation. Such shales have an oily appearance and a strong bituminous odor. Beach fires on Sulphur Island are said to have burned as long as 16 months, although, ordinarily, the shales are incapable of sustaining combustion. The spontaneous distilla

*First Biennial Report, 1861, p. 76.

+Geol. Sur. of Mich. Vol. V. Part Ii, Plate 1. Page 47.

tion of this organic matter has yielded large quantities of petroleum and natural gas, products almost always associated with the stratum, in New York, Ohio, Ontario and Michigan. Crystals of pyrite and nodules of marcasite are of common occurrence in the shales. The latter is especially subject to chemical decomposition, forming compounds of iron and sulphur, which stain the shale and impregnate the percolating water. Spherical to ellipsoidal concretions, varying in size from an inch to six feet in diameter, are found embedded in the mass of shale. These consist according to Rominger, of lime carbonate 89%, magnesium carbonate 2%, with 7.5% of insoluble, bituminous and silicious residue. Crystals of calcite and siderite, sometimes with fragments of organic remains, are found toward the center of each concretion. The shales themselves have yielded but few fossils, either in Michigan or Ohio, and those found are mostly of plants. Rominger reports subordinate seams of lime rock showing "cone-in-cone" structure.

A recent number of the Journal of Geology* contains figures and descriptions of the concretions by Reginald A. Daly. The composition is shown to be approximately the same as that given by Rominger. This author calls attention to the deformation of the shale on all sides of the concretion and concludes that they were formed in place within the shale, that they antedate the period of joint de velopment and final consolidation of the shale, and that the deformation resulted from the energy of the process of crystallization, due to a change in volume when the original bicarbonate of calcium was converted into the monocarbonate.

§ 10. Geological position.

Reposing as these lower beds do upon layers of undoubted Hamil ton, and with the characteristics above presented, it is exceedingly probable that they are to be correlated with the Genesee shale of central New York. The description of lithological characters would apply as well to the shales about Canandaigua Lake as to those of Thunder Bay. That they cannot be regarded as Marcellus, as has been maintained by some, is proven by their position above the Hamilton beds. As yet, however, the lower portion of the St. Clair has furnished no fossils with which to strengthen this lithological evidence of equivalency with the Genesee. For the last forty years the views of those who have worked in this lake region are not in

*February-March, Vol. VIII, No. 2, pp. 135-150. "The Calcareous Concretions of Kettle Point, Lambton County, Ontario.'