SSS Baron ITumboldt on Roch Formations. 
Mexico, in a part of the world so celebrated for the frequency 
of volcanoes, it seems to me, that the chief site of subterranean 
fires is in the transition rocks, and, beneath those rocks. I^have ob- 
served, that all the burning or extinct craters of the Andes open 
in the midst of trap porphyries or trachytes, (Berk Abhandl. 
der Kdn. Acad., 1813, p. 131), and that these trachytes are 
connected with the great transition porphyry^ and syenite forma- 
tion. According to this observation, it appears more natural to 
me to make the secondary and volcanic deposites to follow the 
transition deposite in a parallel manner, and as by bisection. By 
this new arrangement, the formation of porphyries, syenites and 
greywackes, or that of transition porphyries, syenites and granites, 
occurs connected at the same time ; 1^/, With the porphyries of 
the red sandstone in the secondary coal-deposite; With the 
trachytes or trap porphyries wliich are destitute of quartz, and 
mixed with pyroxene. I employ with regret the term volcanic 
terrain^ not that I doubt, like those who designate the trachytes, 
basalts and phonolites (porphyrschiefer), by the name of trap 
terrain.^ that all which I have associated in the volcanic terrain 
has not been produced or altered by fire ; but because several 
rocks, intercalated between the (primitive transition and se- 
condary rocks, might also be volcanic. I would rather wish to 
avoid every (historical) idea of the origin of tilings, in a (sta- 
tistical) view of relative situation or superposition. At Skeen, 
in Norway, a basaltic and porous syenite, containing pyroxenes, 
is placed, according to the observation of M. de Buch, between 
die transition limestone and the syenite with zircons. It is a 
bed, not a dike ; and this is a much less problematical pheno- 
menon than the basalt (Urgrlinstein, Buch. Geogn. Beob. p. 
124, and Baumer, Granit des Riesengebirges^ p. 70.) contained 
in the mica-schist of Krobsdorf in Silesia. The trachytes, with 
obsidian of Mexico, are intimately connected with the transition 
porphyries which alternate with syenites. The amygdaloid be- 
longing to the red sandstone, assumes, on the Continent of 
Europe, and in Equinoctial America, all the appearance of an 
amygdaloid of the basaltic formation. M. Boue, in his interest- 
ing Fssai Geologique siir VEcosse, p. 126, 162, has described 
pyroxenic rocks (dolerites) included in the red sandstone. With- 
out prejudging any thing regarding the origin of these masses, 
