Baron Humboldt on Rock Fo7’mations. 
geographical name has been selected j after having acquired a 
perfect certainty regarding their position. Circumstances are 
the same with regard to the relative age of the molasse of Ar- 
govia, (nagelfluhe and of the Pirna quadersand stein (gres blanc 
of M. de Bonnard), two rocks of Very recent origin, which have 
been very well examined separately, but whose relations to each 
other, and to the chalk and Jura limestone, have only been illus- 
trated of late. One may therefore be pretty sure of having met 
in the New Continent with rocks identical with the molasse or 
quadersaiidstein, without being able to pronounce with certain- 
ty upon their relations with all the secondary or tertiary rocks. 
When rocks are not immediately in contact, and are not covered 
by deposites of known position, their relative age can only be 
conjectured from simple analogies. 
The terms of the geognostical series are either simple or com- 
plex. To the simple terms belong the greater number of the 
primitive formations : the granites, gneisses, mica-slates, clay- 
slates, &c. The complex terms occur in greater numbers 
among the transition rocks : there, each formation includes an 
entire group of rocks, which alternate periodically. The terms 
of the series are not transition limestones or greywackes, consti- 
tuting independent formations; they are associations of clay- 
slate, greenstone, and greywacke ; of porphyry and grey wacke ; 
of granular steatitic limestone, and of conglomerates, composed 
of primitive rocks ; of clay-slate and black limestone. When 
these associations are formed of three or four rocks which alter- 
nate, it is difficult to give them significant names,— -names indi- 
cative of the whole composition of the group,— of all the partial 
members of the complex term of the series. It may then assist 
in fixing the groups in the memory, to retrace the rocks which 
predominate in them, without being absolutely wanting in the 
neighbouring groups. It is in this manner that the granular 
steatitic limestone characterises the Tarantaise formation ; the 
greywacke, the great transition formation of the Hartz and of 
the banks uf the Rhine ; the metalliferous porphyries rich in 
hornblende, and almost destitute of quartz, the formation of 
Mexico and of Hungary. If these phenomena of alternation 
attain their maximum in the transition districts, still they are not 
entirely excluded from the primitive and secondary terrain. In 
