Baroji Humboldt on Rock Forynatlons. 41 
generally adopted by the French mineralogists, has been bor- 
rowed from the celebrated school of Werner : it indicates what 
is, not what is supposed to have been. 
In the geognostical description of the globe, we may distin- 
guish different degrees of aggregation of mineral substances, 
simple or compound, according as we rise to more general ideas. 
Rocks which alternate with one another, which are usually asso- 
ciated, and which present the same relations of position, consti- 
tute 2 i formation ; the union of several formations constitutes a 
district or terrain; but these different terms of rocks. Forma- 
tion and Terrain, are employed as synonymous in many works 
of geognosy. 
The diversity of the rocks, and the relative disposition of the 
beds which form the oxidised crust of the earth, have, from the 
most remote times, fixed the attention of men. Wherever the 
working of a mine was directed upon a deposit of salt, of coal, 
or of clay-iron, which was covered with a great number of beds 
of different natures, it gave rise to ideas more or less precise re- 
garding the system of rocks peculiar to a district of small ex- 
tent. Furnished with these local details, and full of prejudices 
which arise from custom, the miners of a country would disperse 
themselves over the neighbouring districts. They would do 
what geognosts have often done in our days ; they would judge 
of the position of rocks of whose nature they were ignorant, ac- 
cording to imperfect analogies, according to the circumscribed 
ideas which they had acquired in their native country. This 
error must have had a fatal influence upon the success of 
their new researches. In place of examining the connection 
of two contiguous districts, by following some generally ex- 
tended bed, — in place of enlarging and extending, so to speak, 
the first type qf formations which had remained impressed 
upon their minds, — they would be persuaded that each por- 
tion of the globe had an entirely different geological constitu- 
tion. This very old popular opinion has been adopted and sup- 
ported, in different countries, by very distinguished men ; but 
since geognosy has been elevated to the rank of a science, the 
art of interrogating nature brought to perfection, and journeys 
made into distant countries, have presented a more exact com- 
parison of different districts, great and immutable laws have 
been discovered in the structure of the globe, and in the super- 
