2S6 Baron Humboldt on Roch Formations. 
both of these terrains, complex terms are mixed with the simple 
terms of the geognostical series. I shall mention among the se- 
condary formations, the sandstone placed below the alpine lime- 
stone, (the Nebra sandstone, bunte sandstein), which is an asso- 
ciation of marly clay, sandstone and oolites ; the limestone which 
covers the red-sandstone of the coal-formation (the zechstein or 
alpen-kalkstein), which is a less constant association of limestone, 
of (muriatiferous) gypsum, of stinkstone and of pulverulent 
bituminous marl. In the primitive class we find the three 
first terms of the series ; the oldest rocks either isolated, or al- 
ternating two and two, according as they are geognostically 
more approximated by their relative age, or the whole three al- 
ternating. The granite sometimes forms constant associations with 
the gneiss, and the gneiss with the mica-slate. These alterna- 
tions follow particular laws : we see, (for example in Brazil, and, 
although less distinctly, in the maritime chain of Venezuela),, the 
granite, gneiss and mica-schist in a triple association ; but I have 
not found granite alternating alone with mica-slate, or gneiss and 
mica-slate alternating by themselves with clay-slate. 
We must not confound, and on this point I have often in- 
sisted in the present article, rocks passing insensibly to those 
which are in immediate contact with them ; for example, mica- 
slates, which oscillate between gneiss and clay-slate, with rocks 
which alternate with one another, and which preserve all their 
distinctive characters of composition and of structure. M. 
D’Aubuisson has long ago shewn how chemical analysis approxi- 
mates the clay-slate to mica. ( Journal de Physique ^ vol. 68. 
p. 15^8. ; Traite de Geognosie, vol. ii. p. 97*) The first, it is 
true, has not the metallic lustre of mica-slate ; it contains a lit- 
tle less potash, and more carbon ; the silex does not unite into 
nodules or thin laminae of quartz, as in the mica-slate ; but it 
cannot be doubted, that scales of mica form the principal base 
of the clay-slate. These scales are so joined together, that the 
eye cannot distinguish them in the mass. It is perhaps this 
same affinity which prevents the alternation of clay-slates and 
mica-slates : for in these alternations Nature seems to favour the 
association of heterogeneous rocks ; or, to make use of a figura- 
tive expression, she delights in the associations whose alternating 
rocks present a great contrast of crystallization, of mixture and 
