44 Baron Humboldt on Rock Formations. 
ble problems, with those fabulous times of thejphysical history 
of our planet. 
In order to render the principles better understood, accord- 
ing to which the following table of the superposition (f rocks is 
constructed, it becomes necessary to premise observations fur- 
nished by the practical examination of different districts. We 
shall begin with remarking, that it is not easy to circum- 
scribe the limits of a formation. The Jura limestone and the 
Alpine limestone, which are separated to a great distance in one 
country, sometimes appear closely connected in another. What 
announces the independence of a formation, as has been very 
justly observed by M. de Buch, is its immediate superposition 
upon rocks of a different nature, and which consequently ought 
to be considered as more ancient. The red sandstone is an in- 
dependent formation, because it is superimposed indifferently 
upon black (transition) limestone, upon mica-slate, or upon 
primitive granites ; but in a country where the great formation 
of syenite and porphyry predominates, these two rocks constant- 
ly alternate. There results that the syenite rock is dependent 
upon the porphyry, and scarcely any where covers by itself the 
transition clay-slate or primitive gneiss. The independence of 
formations does not, besides, by any means exclude the unformi- 
ty or concordance of position ; it rather excludes the oryctognos- 
tic passage of two superimposed formations. The transition dis- 
tricts have very often the same direction and the same inclina- 
tion as the primitive ones ; and yet, whatever approximation 
there may be between their origin, we are not the less warrant- 
ed to consider the anthracitic mica-slate or the grey-wacke, al- 
ternating with porphyry, as two formations independent of the 
primitive granites and gneisses which they cover. The confor- 
mity of position is in no way incompatible with the indepen- 
dence of formations, that is to say, it does not prevent the right 
which one has of regarding a rock as a distinct formation. It 
is because the independent formations are placed indifferently 
on all the older rocks, (the chalk upon the granite, the red sand- 
stone upon the primitive mica-slate), that the assemblage of a 
great number of observations made upon very distant points, 
becomes eminently useful in the determination of the relative 
age of rocks. In order to determine that the zircon-sienite is 
