200 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

materials, and were probably to a considerable extent formed of the
redeposited materials of that formation. They show evidences of
much less age, however, although they have suffered considerably as
the result of denudation. The later deposits 'form well-marked
terraces along the stream channels throughout the western portion
of the Coastal Plain, and also cover much of the lower portion of the
intervening country. In the low land of the extreme southeastern
section of the state they bury from view all of the underlying strata.

The relatively small amount of denudation which the Columbia
deposits have suffered as a whole, compared with the earlier formations,
renders it possible to detect three distinct phases, which have been
described as the fluvial phase, the inter-fluvial phase, and the low-
level phase; the first or fluvial phase reaching the fullest develop-
ment along the leading water-ways and their larger tributaries and
consisting in its lower horizon of coarse pebbles and boulders, passing
upward into a brownish loam; the second or inter-fluvial phase being
found typically represented in the country which lies between the
water-ways and characterized by materials of local origin and pro-
duced largely by wave action, although frequent gradations into the
fluvial phase are to be found toward the leading water-ways or at
points where the currents have transported large amounts of river-
derived materials; the last or low-level phase being developed through-
out the area of complete submergence, beyond the action of the
streams, where more regularly stratified deposits of sands, clays and
loams abound. It is this phase which is regarded as coating, with
greater or less thickness, the great area of extremely low country
which forms the eastern portion of the Coastal Plain.

REVIEW OF GEOLOGICAL CHRONOLOGY AS REPRESENTED IN MARYLAND.

In the earliest days of the earth's existence of which we can take
cognizance, when as yet the oldest strata which carry evidence of life
had not been laid down, the continent of North America was but
roughly outlined. There was above the ocean level, first of all, a
great Y of crystalline rock whose apex was in the Adirondack mount-
ains in New York, while its two arms reached one toward Alaska and