TOLCANIC STBATTJM OF OKTia. 
401 
The small volcanic stratum of Ortiz (lat. 9“ 28' — 9° 36') 
formed the ancient shore of the vast hasin of the Llanos ot 
Venezuela: it is composed on the points where I could 
examine it, of only two kinds of rocks,, namely, amygdaloid 
and phonolite. The greyish blue amygdaloid contains feu- 
dillated crystals of pyroxene and mesotype. It forms balls 
with concentric layers of which the flattened centre is nearly 
as hard as basalt. Neither olivine nor amphibole can be 
distinguished. Before it shews itself as a separate stra- 
tum, rising in small conic hills, the amygdaloid seems to 
alternate by layers with the diorite, which we have men- 
tioned above as mixed with carburetted slate and amphibolic 
serpentine. These close relations of rocks so different in 
appearance, and so likely to embai-ass the observer, give 
great interest to the vicinity of Oitiz. If the masses of 
diorite and amygdaloid, which appear to ns to be layers, are 
very large veins, they may be supposed to have been formed 
and upheaved simultaneously. We are now acquainted 
with two formations of amygdaloid ; one, the most common, 
is subordinate to the basalt : the other, much more rare,* 
belongs to the pyroxcnic porphyry.t The amygdaloid of 
Ortiz approaches, by its oryctognostic characters, to the 
former of those formations, and we are ahnost sui’prised to 
find it joining, not basalt, but phonolite,^ an eminently 
felspathic rock, in which we find some crystals of amphi- 
bole, but pyroxene very rarely, and never any olivine. The 
Cerro de Flores is a hill covered with tabiilary blocks of 
greenish grey phonolite, enclosing long crystals (not fendil- 
lated) of vitreous felspar, altogether analogous to the 
phonolite of Mittelgebirgo. It is surrounded by pyroxenie 
amygdaloid ; it would no doubt be seen below, issuing 
immediately from gneiss-granite, like the phonolite of 
* We find e.’tatnples of the latter in Norway (Vardekullen, near Skeen), 
in the mountains of (he Thuringerwald ; in South Tyrol; at Hefeld in 
the Hartz, at Bolanos in Mexico, &c. 
t Black porphyries of M. von Buch. 
X TliPre are phonolites of basaltic strata (the most anciently known) 
and jtiionolites of tr^chytic strata (Andes of Mexico). The former are 
generally above the basalts ; and the extraordinary development of felspar 
in that union, and *he want of pyroxene, have always appeared to me 
very remarkable phenomena. 
von. iir. 2 n 
