484 GEOLOGY OF OHIO. 
the hills are clothed with the verdure of June, and the dividing lines 
will be as sharp and well-defined as if the woods had been laid out and 
planted by the art of the landscape gardener. 
GEOLOGICAL STRUCTURBE. 
Although the entire surface of the county is covered deep in drift, or 
its derivatives, yet the upheaval of the center exposes three formations 
of rock, and there is good reason tosuppose that a fourth would be visible 
but for the immense deposits of gravel in the Miami Valley. These for- | 
mations are Huron shale or black slate, shown im the hills about the 
heads of Mad River, the Corniferous limestone, best seen in the Belle- 
fontaine, Macachack and Middleburg quarries, and the water-lime rock, 
exposed in one place on the Machachack, and in numerous ones in the 
meighborhood of Belle Centre and Northwood, while it is the Niagara 
that is supposed to lie under the drift in Miami Valley. 
The Huron Shale, lying highest, and being from its soft, laminated 
structure most subject to the wear of the elements, has been cut down 
by frost and water until only two irregular islands are left, where out- 
lines are shown, approximately, on the map. 
The smaller of these islands, lying directly east of Bellefontaine, im 
Bush Creek, Lake, and Jefferson townships, is the last outlier of its for- 
mation east of the anticlinal axis of the State, or rather, it is directly 
on the crown of the arch. Its northern end is hidden under the Drift, 
but must lie some where near Harper, and the southern is found about 
three miles south-west of Zanesfield, where a deep cut was begun through 
it some years since om the line of the Delaware Railroad, giving a length 
of about nine miles, with an average width of some two and one-half or 
three miles. 
The second and larger island lies east of Zanesfield and West habe 
and underlies Pickreltown and Wickersham’s Corners, in Rush Creek, 
Jefferson, Perry, Monroe, and Zane townships, with a spur extending 
into the northern edge of Champaign county. It is about twelve miles 
long by three wide, and within its limits are to be found the finest and 
most characteristic exposures. 
‘The thickness of the slate om the line of section A Bis 3110 ee under 
the western or Hogue’s summit, by actual measurement with the level, 
‘and 136 feet, by seca barometrical estimate, under Wickersham’s 
Corners. 
Immediately below these Huron Shale islands lies one large island of 
Corniferous limestone, which ean be traced through Rush Creek, Jeffer- 
son, Perry, Zane, Monroe, Liberty, Lake, Harrison, and McArthur town- _ 
