40 Baron Humboldt on Rock Formations. 
The reddish line is, in relation to the other parts of the spec- 
trum, very bright in the spectra of light produced by the flame 
6f hydrogen gas and alcohol. In the spectrum of the flame of 
sulphur, it is seen with difliculty. 
I intend to repeat the experiments relative to the perfection of 
the Achromatic Telescope, with a new instrument, by which I 
expect to obtain a degree of accuracy more than double of that 
of which the preceding observations are susceptible. I shall add 
to them new experiments made with the same instrument, and for 
which the instrument already described was not adapted. These 
last may perhaps be of great interest in practical and physical 
optics. 
In making the experiments of which I have spoken in this 
memoir, I have considered principally their relations to practi- 
cal optics. My leisure did not permit me to make any others, 
or to extend them farther. The path which I have traced out 
in this memoir, may furnish interesting results in physical op- 
tics ; and it is therefore greatly to be wished that skilful natural 
philosophers would condescend to give them some attention. 
Aet. VI . — On Rock Formatims. By Baron Alexander 
Humboldt *. 
HE word formation designates, in Geognosy, either the man- 
ner in which a rock has been produced, or an assemblage of 
mineral masses, which are so connected together, that they are 
supposed to have been formed at the same period, and present, 
in the most distant parts of the world, the same general rela- 
tions of situation and position. It is thus that the formation 
of obsidian and basalt is attributed to subterranean fires ; and 
thus also that we say the formation of transition clay- slate con- ^ 
tains lydian-stone, chiastolite, alum-slate, and alternating beds of 
black limestone and porphyry. The first acceptation of the 
word is better adapted to the genius of the language ; but it 
has relation to the origin of things, to an uncertain science found- 
ed upon geogonic hypothesis. ' The second acceptation, now 
* Translated from Essai Gcognostiqiic par Alexandre de Humboldt. 
