644 



balls with concentric layers, of which the flat- 

 tened centre is nearly as hard as basalt. Nei- 

 ther olivine nor amphibole can be distinguished. 

 Before it appears like an independent soil, and 

 rises in small conic hills, the amygdaloide 

 seems to alternate by layers with the diorite, 

 which we have seen above mixed with carbu- 

 rated slate, and amphibolic serpentine. These 

 close relations of rocks so different in appear- 

 ance, and so fitted to embarrass the geognost, 

 give a great interest to the vicinity of Ortiz. 

 If the masses of diorite and amygdaloide which 

 appear to us to be layers, are very large veins, 

 they may be supposed to be formed and 

 heaved up simultaneously. We are now ac- 

 quainted with two formations of amygdaloide; 

 one, the most common, is subordinate to ba- 

 saltic soil ; the other, much more rare % belongs 

 to pyroxenic porphyry -f*. The amygdaloide of 

 Ortiz draws near, by its oryctognostic charac- 

 ters, to the former of those formations, and we 

 are almost surprised to find it fixed, not to 

 basalt, but to phonolite^:, an eminently feld- 



* We find examples of the latter in Norway (Vardekullen, 

 near Skeen), in the mountains of Thuringerwald, in South 

 Tyrol, at Ilefeld in Harz, and at Bolanos in Mexico, &c. 



+ Black porphyries of M. de Buch. 



4 There are pholanithes of basaltic soil (the most an- 

 tiently known) and phonolithes of trachytie soil (Andes of 

 Mexico). See my Geogn. Essay, p. 347. The former are 



