48 Baron Hunibolclt on Mock Fonnatums. 
or granitic nature. These slates become at first greenish and 
harder. In proportion as the amorphous paste receives horn- 
blende^ it passes into those hornblendic traps which in former 
times were confounded with basalt. In other cases, the mica, 
which is at first concealed in the amorphous paste, becomes de- 
veloped, and separates into distinct and clearly crystallized 
spangles ; at the same time, the felspar and quartz become visi- 
ble ; and the mass assumes a granular aspect, with very elon- 
gated grains : this is a true transition gneiss. By degrees, the 
grains lose their common direction ; the crystals arrange them- 
selves around many centres ; the rock becomes a transition gra- 
nite or syenite. In other cases, the quartz alone is developed; 
it augments, and becomes rounded into nodules, and the slate 
passes to the best characterized grey-wackes. By these certain 
signs, geognosts, to whom the appearances of nature have become 
familiar by long examination, become aware beforehand of the 
proximity of granular, granitic and arenaceous rocks. Analo- 
gous passages of primitive mica-slate to a porphyritic rock, and 
the return of this rock to gneiss, are observed in the eastern parts 
of Switzerland. (See the luminous developments given by M. 
de Raumer, Fragmented p. 10. and 47. ; M. Leopold de Buch, 
in his Voyage de Claris d Chiavennad fait ml803, and inserted 
in the Magaz, der Berl. Naturf.d vol. iii. p. 115). But these pass- 
ages are not always insensible and progressive; the rocks often 
also succeed each other quickly, and in a very abrupt manner ; 
often (for example, at Mexico, between Guanaxuato and Ovex- 
eras), the limits between the slates, the porphyries and syenites, 
are as distinct as the limits between the porphyries and lime- 
stones ; but even in this case, geognostical relations with the su- 
perimposed rocks are indicated by additional heterogeneous 
beds. It is thus that the transition granite of the syenitic for- 
mation presents beds of basanite, by becoming charged with 
hornblende : it is thus, also, that these same granites sometimes 
pass to euphotide. (Buch, Voyages en NorwegSd vol. i. p. 138., 
vol. ii. p. 83.) 
There results from these considerations, that the mechanical 
analysis of amorphous pastes, by means of demi-triturations and 
washings, (an analysis of which, M. Fleuriau de Bellevue made 
the first attempt, that was crowned with success ; Journ. de 
1 
