Baron Humboldt on Rock Formailons. 51 
superimposed rocks succeed each other, from below upwards, aa 
ill those ideal sections which I designed in 1804, for the benefit, 
of the Mexican School of Mines, and of which many copies]^ 
have been distributed since my return to Europe. ( BosctUejo 
de una Pasrgrafia geognostica, con tahlas que ensenan la es- 
tratificacion y el yamllelismo de las rocas en ambos Continentcs, \ 
■para el uso del Real Seminaino de Mineria de Mexico.) These 
Pasigraphic tables united to my own observations made in both 
Americas what had at that period been known with precision re- 
garding the relative position of the primitive, intermediary and 
secondary rocks in the Old Continent. They presented, together 
with the type which might be considered as the most general, 
the secondary types, that is to say, the beds which I have named 
parallel. This same method has been followed in the work 
which I now publish. My parallel formations are geognostical 
equivalents ; they are rocks which repres^ent each other. (See the 
Traite de Geologic de M. d? Aubuisso7i, vol. ii. p. 255.) In Eng- 
land, and on the opposite Continent, there does not exist an iden- 
tity of all the formations : there exist equivalents or parallel for- 
illations. That of our coal situated between the transition masses 
and the red-sandstone, the position of the rock-salt which occurs 
on the Continent in the alpine limestone, and the position of our 
oolites in the Nebra sandstone and Jura limestone, may guide 
the geognost in the approximation of remote formations. In 
England, we observe the coals placed upon transition formations ; 
for example, upon the mountain-limestone of Derbyshire and of 
South Wales, and upon the transition sandstone, or old red- 
sandstone of Plerefordshire. I have thought that I recognised 
in the magnesian-limestone the red marl, the lias and white 
oolites of Bath, the united formations of the alpine limestone, 
of the oolitic sandstone and Jura limestone. In comparing the 
formations of countries more or less distant from each other, 
tjiose of England and of France, for instance, of Mexico and 
Hungary, of the secondary basin of Santa Fe de Bogota and of 
Thuringia, we must not think of opposing to each individual 
rock a parallel one ; it must be recollected, that a single forma- 
tion may represent several others. It is according to this princi- 
ple that beds of clay, lying beneath the chalk, may, in France, 
d2 
