Baron Humboldt on Rock Fomiations. 5^31 
the geognost, when he tries to rise to a more general point of 
view, from connecting these clays and the chalk with the Jura 
limestone, and from regarding them as the last strata of this 
great formation, composed of calcareous and marly beds. 
The inferior beds of the chalk (tuffeau) contain ammonites. 
The limestone of the mountain of St Peter of Maestricht indk 
cates, as has already been observed by Messrs Omalius and 
Brongniart (Geogr. Miner., p. 13), the passage of the dialk to 
older secondary limestones. Near Caen, according to tho 
beautiful observations of M. Prevost, the clays beneath the chdk 
contain those same lignites which occur, in greater quantity, in 
the clay which is situated immediately above the chalk : cerites, 
which bring to mind the coarse limestone of Paris, are seen in a 
limestone with trigonias, placed between clays inferior to the 
chalk and the oolitic beds. I do not insist upon these particu- 
lar facts ; I mention them only to prove, by a striking example, 
how, on bringing together facts observed in different points of 
the same country, the great phenomenon of alternation reveals 
to us the connections between formations which at first sight 
appear to have nothing in common. It is the property of those 
beds which alternate with one another, of those rocks which 
succeed each other in periodical series^ to present the most mark- 
ed contrast in the two beds which immediately follow each 
other. In geognosy, as in the different parts of descriptive na- 
tural history, the advantage of classifications of variously gra-* 
duated sections must be recognised, without losing sight of the. 
unity of nature ; and those who have contributed the most to 
the advancement of natural philosophy, have possessed at once 
both the tendency to generalize, and the exact knowledge of a 
mass of particular facts. 
It has been customary to terminate the series of deposites by 
the volcanic rocks, and to make them succeed the secondary and 
tertiary, and even the alluvial deposites. In a system constructed 
upon the principle of relative antiquity, this arrangement seems 
to me to have little to recommend it. It is without doubt the 
case that iithoid lavas are spread over the most recent forma- 
tions, even over beds of gravel ; nor can it be denied that there 
exist volcanic productions of different epochs : but, from what 
I have observed in the Cordilleras of Penq of Quito, and of 
