ll. dle. Brongniart sur ies caracteres Zoologiques, &c. 215 
does not evidently oppose it?” M.B. adopts the latter 
opinion.—He first shews that rocks very different in min- 
eralogical character may be deposited at the same period ; 
as we now see the argillaceous trap-rocks of Vesuvius ; 
and other volcanoes, the calcareous tufas of many springs, 
and the siliceous deposites of Geyser, &c., formed at the 
same period, though very different in character. On the 
contrary, all the organic remains imbedded in them are 
analogous, and have the common character of the gene- 
ration now existing. es 
The circumstances which cause changes in the mineral 
kingdom may be instantaneous in their operation, such as 
earthquakes, inundations, &c. Not so with those, which 
alter the generations of living beings; their action is ex- 
tended through a long course of ages, and their influence 
is scarcely perceptible from one age to another. Plants 
and animals have scarcely varied their characters since 
the earliest periods of history. ‘The geological characters 
derived from analogy of fossils are therefore more perma- 
nent, and of course of more value in determining the pe- 
riod of formation, than characters simply mineralogical. 
Of these, that from the nature of the rocks is the weakest 
—ithose from the relative height of the formation, the 
depth of its ravines, &c., and the inclination of strata, are 
more important; but these may be produced by sudden 
revolutions, like the earthquakes of Calabria, which have 
changed the order and direction of strata, making hori- 
zontal perpendicular, and vice versa; and throwing recent 
formations apparently below the base of the more an- 
cient. ‘The apparent relative height of formations is a 
delusive character, but their real relative height is essen- 
tially important, though still less so, than that from analo- 
gy of fossils. There are some causes of error, even iu 
this, which ought to be carefully appreciated. Climate 
and locality produce changes in the same species, and 
miay cause a difference in fossils of the same period. Cer- 
tain species have survived the great revolutions of the 
globe, and have lived in different periods, and hence they 
are found in distinct formations—and species belonging to 
earlier formations, may by the abrasion of these formations 
have been mixed and imbedded in formations of a much 
nearer period. 
