NAIUEB OE TUB EORMATIOSS. 
377 
Sectiok hi. 
Nature of the Rocks — Relative Age and Superposition of the Rormationa 
Primitive, Transition, Secondary, Tertiary, and Volcanic Strata. 
The preceding section has developed the gcographicar 
limits of the formations, the extent of the direction of the 
zones of gneiss-granite, micaslate-gueiss, clay-slate, sand- 
stone, and intermediary limestone, which come successivelv 
to light. Wo will now indicate succinctly the nature and 
relative age of these formations. To avoid confounding 
facts with geologic opinions, I shall describe these iorma- 
tions, without dividing them, according to the method 
generally followed, into five groups— primitive, transition, 
secondary, tertiary, and volcanic rocks. I was fortunate 
enough to discover the types of each group in a regmii where, 
before I visited it, no rock had been named. The great 
incouvenience of the old classificatioii is that of obliging 
the geologist to establish fixed demarcations, while he is 
in doubt, if not respecting the spot or the immedmte super- 
position, at least respecting the number of the formations 
which are not developed. How can we in many circum- 
stances determine the analogy existing between a limestone 
with but few petrifactions and an intermediary limestone and 
zechstciii, or between a sandstone superposed on a primi- 
tive rock and a variegated sandstone and quadersandstein, 
or finally, between miiriatiferoiis clay and the red marl ot 
Eimland, or the gem-salt of the tertiary strata of Haly f 
AVhen wc reflect on the immense progress made within 
twenty-five years, in the knowledge of the superposition ot 
rocks, it will not appear siirpruing that my present opinion 
on the relative age of the formations of Equinoctial Anierica 
is not identically the same with what I advanced in l»t)t). 
To boast of a stability of opinion in geology is to boast ot an 
extreme indolence of mind ; it is to remain stationary ainids 
those who go forward. Wliat we obseiwe in any one part ot 
the earth on the composition of rocks, tlmir subordinate 
strata, and th^ order of their position, are facts immutabl) 
