16 
HAROLD’S DISCUSSIONS. 
erates. The fragments may be of different sizes and 
shapes, and may consist of limestone, or shells, or 
pebbles. Oolite is a conglomerate composed of small, 
rounded pebbles of limestone, which look like fish 
eggs, hence the name, which means egg stone. 
The coal series is an interesting one, ranging from 
peat and the various grades of hard and soft coal to 
the pure graphite in our drawing pencils. 
Iron is seldom found in its pure metallic form in 
nature; it readily combines with oxygen, and forms 
several oxids, of which rust is the most common illus¬ 
tration. In this form it occurs in the rocks of all 
ages, as the red and brown colors evidence. It has 
been estimated that iron forms five per cent of all 
the rocks. It is frequently found with manganese, 
which gives the rocks a black or purplish color. 
Water containing carbon dioxid coming in con¬ 
tact with iron oxid dissolves it and leaches it out of 
the rocks. When this is washed down into a peat 
bog it forms a light-colored mineral called siderite. 
If the siderite be subjected to heat it changes to limon- 
ite; under more heat the brown limonite changes to 
red hematite ; if the heat continues it forms magnetite. 
The iron taken out of the mines in northern Wiscon¬ 
sin is mostly in the form of hematite and magnetite. 
Note.— Tlie Hiironian and Laurentian rocks, found chiefly 
around Lakes Huron and Superior, though much distorted, show 
considerable evidence of stratification. For this reason Pro¬ 
fessor Dana considered them as a later division of the Arclijean 
era, but recent geologists have placed them in an era by them¬ 
selves, to which they have given the name Algonkian. 
