ISLAND OF OP.ACIOSA. 
35 
without any appearance of vegetation. On the African soil 
excessive heat and lengthened drought retard the growth of 
cryptogamous plants. 
The basalts of Graciosa arc not in columns, but are 
divided into strata ten or fifteen inches thick. These strata 
are inclined at an angle of SO degrees to the north-west. 
The compact basalt alternates with the strata of porous 
basalt and marl. The rock does not contain hornblende, 
but great crystals of foliated olivine, which have a triple 
cleavage.# This substance is decomposed with great diffi- 
culty. M. Ilaiiy considers it a variety of the pyroxene. The 
porous basalt, which passes into mandelstein, has oblong 
cavities from two to eight lines in diameter, lined with chal- 
cedony, enclosing fragments of compact basalt. I did not 
remark that these cavities had the same direction, or that 
the porous rock lay on compact strata, as happens in the 
currents of lava of Etna, and Vesuvius. The marl,f which 
alternates more than a hundred times with the basalts, is 
yellowish, friable by decomposition, very coherent in the 
inside, and often divided into irregular prisms, analogous 
to the basaltic prisms. The sun discolours their surface, as 
it whitens several schists, by reviving a hydro-carburetted 
principle, which appears to be combined with the earth. 
The marl of Graciosa contains a great quantity of chalk, 
and strongly effervesces with nitric acid, even on points 
where it is found in contact with the basalt. This fact is 
the more remarkable, as this substance does not fill the 
fissures of the rock, but its strata are parallel to those of 
the basalt ; whence we may conclude that botli fossils are of 
tho same formation, and have a common origin. The phe- 
nomenon of a basaltic rock containing masses of indurated 
marl split into small columns, is also found in the Mittelge- 
birge, in Bohemia. Visiting those countries in 1792, in 
company with Mr. Preicsleben, we even recognized in the 
marl of the Stiefelberg the imprint of a plant nearly resem- 
bling the Cerastium, or the Alsine. Are these strata, con- 
tained in the trappean mountains, owing to muddy irrup- 
tions, or must we consider them as sediments of water, 
which alternate with volcanic deposits ? This last hypo- 
thesis seems so much the less admissible, since, from the 
* Blsettriger olivin. t Mergel. 
D 2 
