It occurs mainly in veins and irregular deposits in limestone of Paleozoic Age closely associated with certain igneous rocks.
They existed only during the first half of the Paleozoic era.
Paleozoic means "ancient life;" Mesozoic, "middle life;" Cenozoic, "recent life."
Cambrian , the first period of the Paleozoic era,—that of the first abundance of marine animals.
Indeed, as in North America, the submergence has decreased on the average since the Paleozoic era.
See what it means to tell these kids about the Paleozoic age and sich, Ruthie!
In Europe the wonderful records of Paleozoic history are found in strata, whose estimated maximum thickness is about 100,000 feet!
This is called the "Paleozoic" period, which means the period of "ancient life."
Paleodictyoptera: an ordinal name suggested by Scudder for Paleozoic insects which cannot be assigned to existing orders.
The class Mammalia, to which man belongs, had no representative on the earth during the extended Paleozoic time.
The era of geologic time from about 540 to 245 million years ago. The beginning of the Paleozoic Era is characterized by a great diversity of marine invertebrate animals. Primitive fish and reptiles, land plants, and insects also first appeared during this time. The end of the Paleozoic is marked by the largest recorded mass extinction in the Earth's history, which wiped out nearly 90% of known marine life forms. See Chart at geologic time.