TRUE VOLCANOES. 333 



appears, however, it is entirely destitute of the olivine which 

 occurs so abundantly on the Rio Pisque and near Gualla- 

 bamba, where 1 saw basaltic columns of 72 feet in height 

 And 3 feet thick, which contained both olivine and horn- 

 blende scattered in them. In the dyke of Ansango nume- 

 rous tablets, cleft by weathering, indicate porphvritic slates. 

 All the blocks have a yellowish gray crust from weathering. 

 As the detritus-ridge (called los derrumbamientos, la reven- 

 tnzon, by the natives, who speak Spanish), may be traced 

 from the Eio del Molina, not far from the farm of Pintac, 

 up to the small crater-lakes surrounded by pumice-stone 

 (chasms filled with water), the opinion has grown up natu- 

 rally, and, as it were, of itself, that the lakes are the openings 

 from which the blocks of stone came to the surface. A few 

 years before my visiting the district, the ridge of fragments 

 was in motion for weeks upon the inclined surface, without 

 any perceptible previous earthquake, and some houses near 

 Pintac were destroyed by the pressure and shock of the 

 blocks of stone. The detritus-ridge of Ansango is still with- 

 out any trace of vegetation, which is found, although very 

 sparingly, upon the two more weathered and certainly older 

 eruptions of the plateau of Antisana. 



How is this mode of manifestation of volcanic activity, 

 the action of which I am describing, to be denominated? 24 

 Have we here to do with lava-streams ? or only with semi- 

 scorified and ignited masses, which are thrown out uncon- 

 nected, but in chains pressed closely upon each other (as on 

 Cotapaxi in very recent times)? Have the dykes of Yana 

 Volcan and Ansango been perhaps merely solid fragmentary 

 masses, which burst forth without any fresh elevation of 

 temperature from the interior of a volcanic conical mountain, 

 in which they lay loosely accumulated and therefore badly 

 supported, their movement being caused by the concussion 

 of an earthquake, impelled by shocks or falls and giving rise 

 to small local earthquakes ? Is no one of the three manifes- 



24 " There are few volcanoes in the chain of the Andes," says Leopold 

 von Buch, "which have presented streams of lava, and none have 

 ever been seen around the volcanoes of Quito. Antisana, upon the 

 eastern chain of the Andes, is the only volcano of Quito upon whii/j 

 M. de Huniboldt saw, near the summit, something analogous to 7 

 stream of lava ; this stream was exactly like obsidian" (Descr. dea II* 

 Canaries, 1836, pp. 468 and 488). 



