ON VOLCANIC SUBSTANCES. 



299 



Letter to J. C. Delametherie on some Granatoid Lavas, 

 by J. P. D'AuBuissoN*» 



jL Have read with much pleasure in your number for Sep- g^^^ 



tember last the letter of Mr. Cordier, in which this mineral- toid lavas com- 



ooist communicates to you the principal results of his ob- ^"''^^ of feld- 

 ® . ■ . ^ , . spar and hom. 



servatiotis and experiments on volcanic products*. To his blende. 



third assertion, " the porous or massive granitoid lavas of 



the extinct volcanoes in the interior of France are composed 



of feldspar, pyroxene (augite), and titanized iron," I can 



add, *? sonq:e of the lavas too are composed of amphibole 



and feldspar." 



T have in my possession a specimen of lava from Cantal, L^va from 

 which is composed of, 1st, amphibole in long crystals, very Cantal 

 black, perfectly laminar, and exhibiting in the most distinct 

 manner the two directions of the laminae, cutting each other 

 at an angle of about 124°, which, as is well known is the 

 distinguishing characteristic of the amphibole : ^dly, feld- 

 spar in crystals of a vitreous aspect, like that of almost all 

 volcanic products : 3dly, a blackish gray matter perforated 

 with numerous small pores. This matter predominates in 

 the mass ; yet in some places the amphibole is more abun- 

 dant. This lava is a true secondary greenstone ; that is to ary "greoi 

 say, one of tliose found in the secondary trapformation, and stone. 

 which are composed chiefly of amphibole and feldspar. 



If the crystals that constitute the lava of which I speak Passes into a 

 diminished in size so as to be no longer distinguishable by basalt compo- 

 the eye, and the whole ultimately formed a homogeneous mentarv mol»- 

 -mass, which certainly happens in various parts of the stream cule. 

 from which the specimen I possess was broken off, the result 

 would be a compact black rock, a real basaltes, composed 

 only of the elements of amphibole and feldspar, the same 

 greenstone but in a compact state: it would be to it nearly 

 ,the same as compact limestone is to granular. 



f Journal de Physique, vol, LXIII, p. 385. 

 ■t See the preceding aiticle. 



The 



