TRUE VOLCANOES. 415 



more ancient than the basalts ; where they probably never 

 form dikes, but on the contrary dikes of basalt frequently in- 

 tersect the porphyritic schist (phonolite). In the chain of 

 the Andes belonging to Quito I found the basalt formation 

 a great distance apart from the prevailing trachytes ; almost 

 solely at the Rio Pisque and in the valley of Guaillabamba.* 

 As in the volcanic elevated plain of Quito every thing is 

 covered with trachytes, trachytic conglomerates, and tufas, it 

 was my most earnest endeavor to discover, if possible, some 

 point at which it might be clearly seen on which of the older 

 rocks the mighty cone and bell-shaped mountains are placed, 

 or, to speak more precisely, through which of them they had 

 broken forth. Such a point I was so fortunate as to dis- 

 cover in the month of June, 1802, on my way from Rio- 

 bamba Nuevo (9483 feet above the surface of the South Pa- 

 cific), when I attempted to ascend the Tunguragua, on the 

 side of the Cuchilla de Guandisava. I proceeded from the 

 delightful village of Penipe over the swinging rope-bridge 

 (puente de maroma) of the Rio Puela to the isolated hacienda 

 of Guansce (7929 feet), where to the southeast, opposite to 

 the point at which the Rio Blanco falls into the Rio Cham- 

 bo, rises a splendid colonnade of black trachyte resembling 

 pitch-stone. It looks at a distance like the basalt quarry at 

 Unkel. At Chimborazo, a little higher than the basin of 

 Yana-Cocha, I saw a similar group of trachytic columns of 

 greater height, but less regularity. The columns to the south- 

 east of Penipe are mostly pentagonal, only fourteen inches 

 in diameter, and frequently bent and diverging. At the foot 

 of this black trachyte of Penipe, not far from the mouth of 

 the Rio Blanco, a very unexpected phenomenon presents itself 

 in this part of the Cordilleras greenish-white mica-slate with 

 garnets interspersed in it ; and farther on, beyond the shal- 

 low stream of Bascaguan, at the hacienda of Guansce, near 



* See above, p. 313. The Kio de Guaillabamba flows into the Rio 

 de las Esmeraldas. The village of Guaillabamba, near which I found 

 the isolated oliviniferous basalt, is only 6430 feet above the level of the 

 sea. An intolerable heat prevails in the valley, which is still more 

 intense in the Valle de Chota, between Tusa and the Villa de Ibarra, 

 the sole of which sinks to 5288 feet, which is rather a chasm than a val- 

 ley, being scarcely 9600 feet wide and 4800 feet deep (Humboldt, Rec. 

 cf Observations Astronomiques, vol. i., p. 307). The rubbish-ejecting 

 Volcan de Ansango, on the descent of the Antisana, does not belong 

 to the basalt formation at all : it is an oligoclase trachyte resembling 

 basalt (compare, for the distances, Antagonisme des Basaltes et des Tra- 

 chytes, rny Essai Geognostique sur le Gisement des Roches, 1823, p. 348 

 and 359, and generally, p. 327-336). 



