Page:South African Geology - Schwarz - 1912.djvu/135

 has the number of geologists who have studied the questions been sufficiently great, for the perplexities to have been unravelled.

The sedimentary rocks of Europe and America are divided into four main groups, on the principles laid down by Werner. These are now classed according to whether they have no fossils, or whether the fossils are the remains of primitive type of animals, or of animals intermediate between these and the recent forms, or, finally, of animals nearly allied to recent forms. The actual terms are Azoic, with no life; Palæozoic, with ancient life; Mesozoic, with intermediate life; Cainozoic, with recent life. If we wish to speak of the time during which the group of rocks was deposited, we use the term era; thus the Mesozoic Era is the time when the great reptiles lived on land and ammonites lived in the sea.

Groups are divided into Systems, corresponding with Periods of time.

Systems are divided into Series, corresponding with Epochs of time.

Series are divided into Stages, corresponding with Ages of time.

Stages are divided into Zones, corresponding with Hemerse of time.

Zones are divided into Strata or individual Beds.

In South Africa we have two main groups of beds; to the newer one, because it extends from the Palaeozoic to the Mesozoic, we cannot apply either of the European group names; and an older group, which, though being in the literal sense " Azoic " as far as investigation has gone, may at any time yield fossils which might put the particular system into the Ordovician or Cambrian. Two separate group names are thus requisite: the newer