LAPSE OF TIME, 19 
The tabular view given on page 15 shows the succession 
of the palzontological rock-groups, systems, and formations, 
that is, the larger and smaller neptunic groups of strata, 
which contain petrifactions, from the uppermost, or Alluvial, 
down to the lowest, or Laurentian, deposits. The table on 
page 14 presents the historical division of the correspond- 
ing eras of the larger and smaller paleontological periods, 
and in a reversed succession, from the most ancient Lauren- 
tian up to the most recent Quaternary period. 
Many attempts have been made to make an approximate 
calculation of the number of thousands of years constituting 
these periods. The thickness of the strata has been compared, 
which, according to experience, is deposited during a century, 
and which amounts only to some few lines or inches, with 
the whole thickness of the stratified masses of rock, the 
succession of which we have just surveyed. This thickness, 
on the whole, may on an average amount to about 130,000 
feet; of these 70,000 belong to the primordial, or archilithic ; 
42,000 to the primary, or palzolithic; 15,000 to the secondary, 
or mesolithic; and finally only 3,000 to the tertiary, or 
cenolithic group. The very small and scarcely appreciable 
thickness of the quaternary, or anthropolithic deposit 
cannot here come into consideration at all. On an average, 
it may at most be computed as from 500 to 700 feet. 
But it is self evident that all these measurements have only 
anaverage and approximate value, and are meant to give 
only a rough survey of the relative proportion of the 
systems of strata and of the spaces of time corresponding 
with them. 
Now, if we divide the whole period of the organie history 
of the earth—that is, from the beginning of life on the earth 
