634 _ STRATIGRAPHICAL GEOLOGY. [Book VI. 
succession,” might be tempted to speculate about the extinction of 
these organisms, and their replacement by other and later forms of 
life, such as the ferns, lycopods, ganoid fishes, and other fossils so 
abundant in the overlying strata. But further research would show 
him that, high above the plant-bearing sandstones and coals, other 
limestones and shales might be observed, charged with the same 
marine fossils as before, and still further overlying groups of sand- 
stones, coals, and carbonaceous beds followed by yet higher marine 
limestones. He would thus learn that the same organisms, after 
being locally exterminated, returned again and again to the same 
area. Such a lesson would probably teach him to pause before 
too confidently asserting that the highest bed in which certain 
fossils can be detected, marks really their final appearance in 
the history of life. An interruption in the succession of fossils 
may thus be merely temporary or local, one set of organisms 
having been driven to a different part of the same region, while 
another set occupied their place until the first was enabled to 
return. 
7. The Geological Record is at the best but an imperfect 
chronicle of the geological history of the earth. It abounds in gaps, 
some of which have been caused by the destruction of strata owing 
to metamorphism, denudation, or otherwise, some by original non- 
deposition, as above‘explained. Nevertheless it is from this record that 
the progress of the earth is chiefly traced. It contains the registers of 
the births and deaths of tribes of plants and animals which have from 
time to time lived on the earth. Probably only a small proportion 
of the total number of species which have appeared in past time have 
been thus chronicled, yet by collecting the broken fragments of the 
record an outline at least of the history of life upon the earth can be 
deciphered. 
It cannot be too frequently stated, nor too prominently kept in 
view, that, although gaps occur in the succession of organic remains 
as recorded in the rocks, there have been no such blank intervals in 
the progress of plant and animal life upon the globe. The march of 
life has been unbroken, onward and upward. Geological history, 
therefore, if its records in the stratified formations were perfect, ought 
to show a blending and gradation of epoch with epoch, so that no 
sharp divisions of its events could be made. But the record of the 
history has been constantly interrupted ; now by upheaval, now by 
yoleanie outbursts, now by depression, now by protracted and 
extensive denudation. These interruptions serve as natural divisions 
in the chronicle, and enable the geologist to arrange his history into 
periods. As the order of succession among stratified rocks was first 
made out in Kurope, and as many of the gaps in that succession were 
found to be widespread over the European area, the divisions which 
experience established for that portion of the globe came to be 
regarded as typical, and the names adopted for them were applied to 

