JOG 
primitive rock in lavas. 
the Azores ; hut it is certain, that the island of Bourbon 
as well asTeneriffe, exhibits only a heap of lavas and 
basalts. Ao volcanic rock rears its head, cither on the 
hros Morne, or on the volcano of Bourbon, or on the colos- 
sal pyramid of Cimandef, which is perhaps more elevated 
than the Peak of the Canary Islands. 
. B ° , 7 Sk Vincent nevertheless asserted, that lavas includ- 
es. fragments of granite have been found on the elevated 
plain oi Petaina ; and M. Broussonnet informed me, that on a 
lull above Guimar, fragments of mica-slate, containing beau- 
tiful plates of specular iron, had been found. I cau affirm 
nothing respecting the accuracy of this latter statement, 
which it would be so much the more important to verify, as 
. ' "° h Naples, is in possession of a fragment of rock 
thrown out by Vesuvius,* which I found to lie a real mica- 
slate. .['very thing that tends to enlighten us with respect 
to tlie site ol the volcanic fire, and the position of rocks 
subject to its action, is highly interesting to geology 
It is possible, that at the Peak of Tenerifle, the' fragments 
oi primitive rocks thrown out by the mouth of the volcano 
may be loss rare than they at present appear to be, and 
may be heaped together in some ravine, not vet visited 
by travellers. In fact, at Vesuvius, these same fra fluents 
are met with only in one siugle place, at the Possa Grande 
where they arc hidden under a thick layer of ashes. If this 
ravine had not long ago attracted the attention of naturalists 
when masses of granular limestone, and other primitive rocks! 
were hud bare by the rains, we might have thought them as 
rare at Vesuvius, as they arc, at least in appearance, at the 
Peak of Teneriffe. 
* T " l ’“ e y® 1 “able collection of Dr. Thomson, who resided at Naples 
; °i 'l 01 kv “ end. sing a real granite, which is composed 
of reddish feldspar with a pearly lustre like adularia, quartz, mica, horn- 
blende, and, what is very remarkable, lazulite. But in general the 
masses of known primitive rocks, (I mean those which perfectly resemble 
our granites, our gneiss, and our mica-slates) arc very rare in lavas • the 
substances we commonly denote by the name of granite, thrown out by 
' esuvl » 3 > , are “'*&«* o1 ' nephclitic, mica, and pyrosene. We are igno'- 
rant whether these matures constitute rocks tui generis placed under 
granite, and consequently of more ancient date; or simply form either 
intermediate strata a- veins, in the interior of the primitive mountains, th« 
tops of which appear at the surface of the globe. 
