Baron Humboldt on Rock Formations. 45 
a transition rock, it must have been seen resting upon formations 
posterior to the black limestone with orthoceratites. Observations 
made upon the porphyries and syenites of Hungary by M. Beu- 
dant, one of the most distinguished geologists of the present 
times, may throw much light upon the formations of the Mexi- 
can Andes. It is thus that a new vegetable discovered in In- 
dia, elucidated the natural affinity between two families of plants 
belonging to Equinoctial America. 
The order which has been followed in the table of formations, 
is that of the situation and relative position of rocks. I do not 
pretend that this position is observed in all the countries of the 
globe ; I merely point it out such as it has appeared the most 
probable, after the comparison of a great number of facts which 
I have collected. It is by the idea of the relative age of for- 
mation, that I have been guided in this work, imperfect as it 
still is. I had begun it long before my journey to the Cordille- 
ras of the New Continent, from the year 179S, when, on leav- 
ing the Frey berg School, I was appointed to the direction of 
the Mines in the mountains of the Fichtelgebirge. The same 
rock may vary in composition, integrant parts may have been 
abstracted, and new substances may occur disseminated, without 
the rock’s changing its denomination in the eyes of the geognost 
who is engaged with the superposition of formations. Un- 
der the equator, as in the north of Europe, strata of a true tran- 
sition syenite lose their hornblende, without the mass becom- 
ing another rock. The granites of the banks of the Ori- 
noco sometimes assume hornblende as an integrant part, and 
yet do not cease to be primitive granite, although this may not 
be of the first or oldest formation. These facts have been ob- 
served by all practical geologists. The essential character of 
the identity of an independent formation is its relative position, 
the place which it occupies in the general series of formations. 
(See the classical Memoir of M. de Buch, ‘Ueber den Begriff 
einer.^ in the Mag.Aer Naturf.^ 1810, p. 1^8-138.) It is on 
this account that an isolated fragment, a specimen of rock found 
in a collection, cannot be determined geognostically, that is to 
say, it cannot be referred with certainty to a particular forma- 
tion, constituting one of the numerous beds of which the crust 
of our planet is composed. The presence of chiastolite, the ac- 
