i08 
THE NATURALISTS' COMPANION. 
RISTOBICAL GEOLOG Y. 
A SERIAL. 
B\' FRANKLIN C. JOMNSON. 
CHAP'rKR III. 
TRENTON PERIOD. 
1,'he Trenton formation extends along 
the Appalachian range, from the Creen 
Mountains to Alabama. It is also 
tound in the north and west, d'he 
Trenton limestone ^ takes ^its name 
from Trenton Falls, on West _^Canada 
Creek, near Utica, Oneida (Munty, N. 
Y. The Galena limestone is included 
in it. 
During this period the seas swarmed 
with life ; but nothing terrestrial has ev¬ 
er been found. A coiled shell, called 
MACLURiE MAGNA, is abundant. Corals, 
crinoids, shells, etc., are found in im¬ 
mense numbers. Trilobites. appear in 
great numbers. A coral, bearing the 
short name of column aria alreolata, 
is very abundant. It is often found in 
great masses of a ton’s weight. It looks 
almost precisely like honeycomb. But 
the rover of the Silurian seas ol this 
period was the Orthoceratite. They 
had a long shell which was divided into 
partitions, somewhat like the shell of 
the Nautilus. They have been found 
with a shell 30 thirty feet long. 
HUDSON PERIOD. 
'Fhis formation is found along the 
Mohawk and Hudson rivers. It is also 
found on Lake Michigan, in the Missis¬ 
sippi valley, and along the Appalachian 
range. The sea is still full of trilobites, 
orthoceratites, corals, shells, etc. A 
radiate, the graptolite (rock-writing) 
is numerous during this period. 
Lake Champlain was made during 
this period. 
NIAGARA PERIOD. 
'Fhis formation is found though the 
Appalachian region. Also in Canada 
and the West. 
Sea weeds are very plentiful, 1 
During this period crinoids became 
abundant. Crinoids are often called 
•‘stbne-liliies” because their cup-shaped 
body much resembles a lillie in shape. 
From their l)odLes there branched out 
live or more long arms, which branched 
out again to sometimes as many as a 
thousand. 'The long stalks were jointed. 
SALINA PERIOD. 
'Fhis period takes its name from the 
Salina salt springs near the city ofSyia- 
cuse, N. Y. 
'Fhe formation runs parallel with the 
Niagara limestone westward to Milwau¬ 
kee. 'Fhe rocks are mostly shales, 
marls, and limestone. Gypsum is com¬ 
mon. 
Fossils are rare. 'Fhe salt water was 
probably unfavorable to animal life. 
LOWER HELDERBERG PERIOD. 
So called from the Helderberg Moun¬ 
tains, near Albany, N. Y. The for¬ 
mation is found in many parts of the 
State of New York, also in Ohio, In¬ 
diana, 'Fennessee, Illinois, Maine, and 
parts of the South. 
Many fossils are found in the lime¬ 
stone of this period. 
A crustacean, the eurypterus is 
found in large numbers. It is some¬ 
what like the tiilobito. Some of them 
attained the length of six feet or more. 
Small slender cones, called tenta- 
GULiTES, which almost compose the mass 
of some of the rocks, are abundant. 
Corals and crinoids were very abund¬ 
ant. 
ORISKANY PERIOD. 
'Fhis formation extends from the town 
oi Oriskany, N. Y., southward to Vii- 
ginia. It is also found in Maine. 
'Fhe fossil most abundant is a brachii.- 
pod, called the spirifer arenosus. 
CHAPTER IV. 
II. DEVONIAN AGE. 
'Fhis is the second great act on the 
stage of life. 
It takes its name from the county of 
Devon, England, where the rocks are 
