MARYLAND GEOLOGICAL SURVEY 155

of geological history called Archean Time. These oldest rocks *are
largely crystalline in character, so that there can be but little chance
of encountering organic forms, even had they earlier existed in the
strata. Even the least altered deposits, although they have afforded
a few scattered remains of archaic forms at certain points, contain
nothing more than the merest traces of the organisms of this early
time.

When, however, life does once appear in all its variety, it is well
nigh the same in all the older rocks. In the most widely separated
localities the same types recur in rocks of the same age, and this
furnishes us with the key to the succession of deposits. From the
time when the oldest fossil-bearing stratum was deposited until now,
the story of life-progress and development is told by the rocks with
sufficient clearness to be unmistakable. Local differences of condi-
tions have probably always prevailed, as they do now, but the same
types of organisms have always lived at the same time over the entire
globe, so that their remains serve as sufficient criteria for the correla-
tion of the strata, which contain them. The sequence of life-forms
once made out gives us, for the whole earth, the means for fixing the
order of deposits even when this is most profoundly disarranged by
foldings of the strata into mountains or by other earth movements.

Geologists distinguish three principal divisions in the history of life
as read in the record of the rocks. During the earliest of these great
time-divisions, archaic forms of life flourished—uncouth fishes, crust-
aceans, mollusks, and tree-ferns—most of them very unlike those now
extant. On this account this is known as the period of most ancient
life or Paleozoic Time. To this succeeded a long lapse of ages when
enormous reptiles predominated, associated with other types more like
those that now inhabit the globe. To this is given the name of middle
life or Mesozoic Time. Finally living things began to assume the
form and appearance with which we are familiar, so that this last
grand time-division, which includes the present, is designated as the
period of recent life or Cenozoic Time.

Each of these three grand divisions of geologic time is in its turn
separated into shorter subdivisions called Periods, characterized by