Royal Institute of France. 461 
finally, are completely in this state: but there always remain so 
many traces of their origin that we cannot mistake them. They 
are always referred to one of the two principal orders of combi- 
nations recognised among the stony lavas. 
M. Cordier endeavours to explain, by the difference in the 
state of the scoriz, the phenomenon which has astonished so 
many travellers; namely, that certain currents of lava remain 
constantly sterile, while others are soon covered with the most 
luxuriant vegetation. It arises from the circumstance of the 
former being more vitrified than the latter, and therefore being 
less easily decomposed. 
The author also examines the obsidians, or volcanic glass; 
and by comparing together all the shades of their greater or 
lesser vitrification, he always finds some traces of that pyroxene 
or feldspar, the predominant principles of the two orders of lava 3, 
and the obsidians which melt into black glass have shown per- 
fect’ transitions even the length of the thickest basalt; in a 
word, the obsidians, the seoriz, the lava, the basalts do not 
differ in composition, but only by the accidents in their texture. 
Even the volcanic sands and ashes yield upon being washed the 
same materials the aggregation of which forms the adjacent 
lavas. M. Cordier has pursued those materials into various 
substances, and after they had been altered by time, and extri- 
cated them from the new substances which enveloped them, or 
which had been as if filtered into the interstices. In a word, he 
has not neglected the examination of any of the modifications 
of the true or contested volcanic productions, and has no 
where found his general rules defective; but when he passed af- 
terwards to those trapps, and petro-silex, in short, to those an-. 
cient rocks to which basalts are sometimes referred, he has no 
longer recognised any of those characters so marked, which are 
said to establish between the lavas and the basalts incontestable 
relations. 
The mass of those ancient rocks has no apparent vacuities, 
and they do not differ from each other in point of colour. This 
cannot be insulated, nor can a mechanical analysis be made of 
them: consequently, if a part ofthose rocks are composed of 
heterogeneous materials, it is not possible to determine the mi- 
neralogical species to which those materials belong. 
Their chemical analysis also gives other results, particularly. 
because it yields no titanium. 
Thus the pretended analogy between the trapps and the ba- 
salts will not support a rigorous examination. 
As to the origin of the lavas and the causes of their fusion, 
M. Cordier risks no conjecture ; “but, considering their mass as 
coagulated by an instantaneous crystallization, he easily resolves 
the 
