REVIEW OF GEOLOGICAL STRUCTURE. 7 
containing lime and phosphorus, but generally little silica or alumina. 
In Tennessee, where it is called the dye-stone ore, much of it is quite 
earthy and silicious, but there it was deposited near the shore and is 
mingled with a large amount of sand, washed down and deposited, with 
it. In its most characteristic form the Clinton ore is granular or oolitic; 
consisting of flattened concretions of hematite mingled with innumerable 
fragments of shells and joints of crinoids. In places, too, it contains 
numbers of well marked fossils, and it is everywhere a marine and not 
a marsh deposit. It was probably formed by the precipitation of the 
iron of the drainage waters of the land which bordered the shallow Clin- 
ton Sea on the north and east. This land contained an unusual quantity 
of iron, and any one who would traverse the margin of the basin where 
it was deposited, beginning at Marquette and following the line of the 
Canadian highlands, the Adirondacks, and the Allegheny belt, would pass 
over the most important iron deposit on this continent. The chalybeate 
waters flowing from this shore apparently deposited the iron they carried 
in the form of minute concretions, of hydrated sesquioxyd, just as the 
‘““mustard seed ore’—granular or oolitic Limonite—is now being deposited 
in some of the Swedish lakes, which receive the drainage from ferruginous 
districts. While in the process of transportation the iron was a soluble 
protoxide, but by oxidation was rendered insoluble and precipitated. In 
the ages that have sinee passed these limonite granules have lost their 
water of combination, as all the older limonites have done, and have been 
converted into red hematite or the anhydrous sesquioxyd. They also 
became somewhat flattened by pressure, so as to take the form of flax- 
seed rather than mustard seed, producing what is sometimes known as 
flax-seed ore. 
THE NIAGARA GROUP. 
This is the deposit from the Upper Silurian Sea at the ee of its 
greatest depth and breadth. In Ohio it attains a thickness of about 
three hundred feet,* and consists of several distinct members which show 
that considerable oscillations of level occurred in the Niagara Sea during 
the long period of its continuance. The Niagara shales mark an inter- 
val of comparatively shallow and turbid water; the nearly pure dolo- 
mites—the Springfield and Cedarville limestones—were deposited from 
deeper and purer water ; while the Hillsboro sandstone, which caps the 
*Dana (Manual, page 221) says: ‘‘ The Niagara is in Ohio the lower part of the Cliff 
limestone, and is eighty feet thick.” The term Cliff limestone was vaguely used many 
years since to designate the Corniferous and Upper Silurian limestones, sometimes one 
or the other, sometimes both ; it is now entirely obsolete. The thickness of the Niagara 
Group in Ohio, as given by Dana, is, without doubt, an accidental error. 
