50 Baron HLiiiiboldt on Rocla Formations^ 
to which the orjctognost, who classes rocks according to their 
composition, would give entirely different denominations. These 
remarks have not escaped the learned author of the Classijica-^ 
tion Mineralogique des Roches ; they must have presented them- 
selves to an experienced geognost, who has so successfully in- 
vestigated the superposition of the deposites of which he has 
treated. ‘‘We must not confound,” says M. Brongniart, in his 
late Memoir on the position of the Ophiolites, “ the relative po- 
sitions, the orders of superposition of the deposites and of the 
rocks which compose them, with purely mineralogical descrip- 
tions. The neglect of making the proper distinction in this 
case, would necessarily be productive of confusion in the science, 
and would retard its progress.” The arrangement which we 
give at the end of this article, is by no means what is called a 
classihcation of rocks ; there will not even be found united, un- 
der the title of particular sections (as in the old geognostical 
method of Werner, or in the excellent Traite de Geognosie of 
M. D’Anbuisson), all the primitive formations of granite, nor 
all the secondary formations of sandstone and limestone. It 
has been attempted, on the contrary, to place each rock as it oc- 
curs in nature, according to the order of its superposition or of 
its respective age. The different formations of granite are se- 
parted by gneisses, mica-slates, black-limestones and grey- 
W'ackes. In the transition rocks, we have separated the forma- 
tions of porphyries and syenites of Mexico and Peru, which are 
anterior to the grey-wacke, and to the limestone with orthocera- 
tites, from the much more recent formation of the zircon-porphy- 
ries and syenites of Scandinavia. In the secondary rocks, we have 
separated the oolitic sandstone of Nebra, which is posterior to 
the alpine limestone or zechstein, from the red- sandstone, which 
belongs to the same formation with the secondary porphyry and 
amygdaloid. According to the principle which we follow, the 
same names of rocks occur several times in the same table. An- 
thracitic mica-slate is separated, by a great number of older 
formations, from the mica-slate anterior to the primitive clay- 
slate. 
Instead of a classiheation of granitic, schistose, calcareous and 
arenaceous rocks, it has been my object to present a sketch of 
the geognostical structure of the globe ; a table in which the 
