420 | JOSEPH FOURIER. 
eye. It is necessary also to note this capital fact, that 
each stratum has a well-defined limit ; that no process of 
transition connects it with the stratum which it supports. 
The ocean, the original source of all these deposits, un- 
derwent then formerly enormous changes in its chemical 
composition to which it is no longer subject. 
With some rare exceptions, resulting from local con- 
vulsions the effects of which are otherwise manifest, the 
order of antiquity of the successive strata of rocks which 
form the exterior crust of the globe ought to be that of. 
their superposition. The deepest have been formed at 
the most remote epochs. The attentive study of these 
different envelops may aid us in ascending the stream 
of time, even beyond the most remote epochs, and en- 
lightening us with respect to those stupendous revolutions 
which periodically overwhelmed continents beneath the 
waters of the ocean, or again restored them to their for- 
mer condition. Crystalline rocks of granite upon which 
the sea has effected its original deposits have never ex- 
hibited any remains of life. Traces of such are to be 
found only in the sedimentary strata. 
Life appears to have first exhibited itself on the earth 
in the form of vegetables. The remains of vegetables 
are all that we meet with in the most ancient strata de- 
posited by the waters ; still, they belong to plants of the 
simplest structure,—to ferns, to species of rushes, to 
lycopodes. 
As we ascend into the upper strata, vegetation becomes 
more and more complex. Finally, near the surface, it 
resembles the vegetation actually existing on the earth, 
with this characteristic circumstance, however, which is 
well deserving attention, that certain vegetables which 
grow only in southern climates, that the large palm-trees, 
