Baron Humboldt on Rock Formations. 22^ 
of formations, and to direct the attention more especially to great 
;characteristic features, should be more or less cautiously con- 
ducted, according as one describes the basin of a river, an iso- 
iated province, a great country such as France or Germany, or 
an entire continent. 
The more minute the investigation of districts becomes, the 
more does the connection between formations which appear at 
first perfectly independent, manifest itself by the great pheno- 
menon of alternation ; that is to say, by a periodical succession 
of beds which present a certain analogy in their composition, and 
sometimes even in certain fossil organic bodies. It is thus that, 
in the transition-mountains, for example, in America (at the en- 
trance of the plains of Calabozo), beds of greenstone and eupho- 
tide, in Saxony (near Friedrichswalde and Meissen) the clay-slates 
with glance-coal, the greywackes, porphyries, black limestones, 
and greenstones, constitute, from their frequent and repeated 
alternation.^ a single formation. It often happens that subordi- 
nate beds appear only at the extreme limits of a formation, and 
assume the aspect of an independent formation. The cupreous 
and bituminous marls {Kupferscliiefer)^ which occur in Thu- 
ringia between the alpine limestone izechstein) and the red sand- 
stone {rothes liegende)^ and which have for ages been extensively 
wrought, are represented in several parts of Mexico, of New 
Andalusia, and of Southern Bavaria, by multiplied beds of 
marly clay, more or less carburetted, and included within the 
alpine limestone. Similar circumstances often give to gypsums, 
sandstones, and small beds of compact limestones, the appear- 
ance of particular formations. Their dependence on subordina- 
tion is known by their frequent association with other rocks, by 
their want of extent and of thickness, or by their total suppres- 
sion, which is frequently observed. It must not be forgot (and 
this fact has struck me much in the two hemispheres) that the 
great formations of limestones, for example, the alpine lime- 
stone, have their sandstones^ as the very generally extended 
sandstones have their limestone beds. Thin beds of sandstones, 
of limestones, and of gypsums, characterize, in all the zones, 
the deposites of coal and rock-salt orjrauriatiferous clay (salzthon)^ 
isolated deposites, wliich are most commonly only covered by 
these small local formations. It is by overlooking these consi- 
