During the interval
thus indicated, the deposition of rock must of necessity have
been proceeding more or less actively in other areas. Every
unconformity, therefore, indicates that at the spot where it
occurs, a more or less extensive series of beds must be actually
missing; and though we may sometimes be able to point to these
missing strata in other areas, there yet remains a number of
unconformities for which we cannot at present supply the deficiency
even in a partial manner.
[Illustration: Fig. 18.--Section showing strata of Tertiary age
(a) resting upon a worn and eroded surface of White Chalk (b),
the stratification of which is marked by lines of flint.]
It follows from the above that the series of stratified deposits
is to a greater or less extent irremediably imperfect; and in
this imperfection we have one great cause why we can never obtain
a perfect series of all the animals and plants that have lived
upon the globe. Wherever one of these great physical gaps occurs,
we find, as we might expect, a corresponding break in the series
of life-forms. In other words, whenever we find two formations
to be unconformable, we shall always find at the same time that
there is a great difference in their fossils, and that many of
the fossils of the older formation do not survive into the newer,
whilst many of those in the newer are not known to occur in the
older.
Pages:
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108