YOLCA.NIC STBATUM OF OKT1Z. 401 



The small volcanic stratum of Ortiz (lat. 9 28' 9 36') 

 formed the ancient shore of the vast basin 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 fen- 

 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 embarass the observer, give 

 great interest to the vicinity of Ortiz. If the masses of 

 diorite and amygdaloid, which appear to us 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 pyroxenic porphyry.t The amygdaloid of 

 Ortiz approaches, by its oryctognostic characters, to the 

 former of those formations, and we are almost surprised to 

 find it joining, not basalt, but phonolite,J an eminently 

 felspathic rock, in which we find some crystals of amphi- 

 bole, but pyroxene very rarely, and never any olivine. The 

 Cerro de Elores is a hill covered with tabulary blocks of 

 greenish grey phonolite, enclosing long crystals (not fendil- 

 lated) of vitreous felspar, altogether analogous to the 

 phonolite of Mittelgebirge. It is surrounded by pyroxenic 

 amygdaloid; it would no doubt be seen below, issuing 

 immediately from gneiss-granite, like the phonolite of 



* We find examples of the latter in Norway (Vardeknllen, near Skeen). 

 in the mountains of (he Thuringerwald ; in South Tyrol; at Uefeld in 

 the Hartz, at Bolanos in Mexico, &c. 



f Black porphyries of M. von Buch. 



j There are phonolites of basaltic strata (the most anciently known) 

 and phonolites of trachytic 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. 



VOL. III. 2 D 



