LAPSE OF TIME. 1 9 



The tabular view given on page 15 shows the succession 

 of the palseontological 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 palseontological 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 palseolithic; 15,000 to the secondary, 

 or mesolithic ; and finally only 3,000 to the tertiary, or 

 csenolithic 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 

 an average 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 organic history 

 of the earth — that is, from the beginning of life on the earth 



