^28 Baron Humboldt on Rock Formations. 
derations, which should be familiar to every practical geologist, 
that the type of the great independent formations has been ren- 
dered too complicated. 
The phenomenon of alternation manifests itself, either locally 
in rocks, superimposed several times upon each other, and con- 
stituting a single compound formation, or in the series of forma- 
tions considered in their aggregate. It is either greenstones and 
syenites, slates and transition limestones, beds of limestones and 
of marl, that alternate immediately, or the whole is a system of 
mica-slates, and of granular feldspathic rocks (granites, gneisses, 
and syenites), which reappear among the transition deposits, 
and which separate from the primitive homonymous ^system the 
greywackes and limestones with orthoceratites. For the first 
knowledge of this fact, one of the most important and least 
studied of modern geognosy, we are indebted to the beautiful 
observations of Messrs Leopold von Buch, Brochant, and Hauss- 
mann. From the circumstance that, in the transition system, 
granular rocks, perfectly destitute of organic remains, succeed 
to compact rocks which contain these same remains, it has been 
concluded by geognosts of great name, that this alternation of 
shelly and not-shelly rocks, might extend beyond the deposites 
which we call primitive. It has not been merely asked if the 
clay-slates, miea^slates, and gneisses, support the granites which 
have been considered as the oldest; the question has also been 
agitated, whether greywackes and black limestones with madre- 
pores might not recur beneath those same granites. According 
to this idea, the primitive and transition rocks would only form 
a single deposite ; and the first might be regarded as intercalated 
in a deposite posterior to the development of organic beings, 
and which might penetrate to an unknown depth into the inte- 
rior of the globe. I confess, that no direct observation can be 
as yet adduced in support of these opinions. The fragments of 
rocks which I have seen contained in the lithmd lavas of the vol- 
canoes of Mexico, Quito, and Vesuvius, and which are thought 
to have been torn from the bowels of the earth, seem to belong 
to altered rocks of granite, inica^slate, syenite, and granular 
hmestone, and not to greywackes and limestone with madre- 
pores. 
We have preserved in the arrangement of rocks the great 
divisions known by the name of primitive, transition or inter- 
