312 REACTION OF THE INTERIOR OF THE EARTH 



slabs and blocks are partly scoriaceous, and even puffed 

 up like foam at their edges, and partly weathered and 

 mixed with earthy detritus. 



Analogous, but more complex, phenomena are pre- 

 sented by another, also band-like, assemblage of rocks. 

 On the eastern declivity of Antisana, nearly 1300 feet 

 lower in vertical height than the plain of the Hacienda, 

 in the direction of Pinantura and Pintac, there are two 

 small circular lakes, of which the northernmost is called 

 Ansango, and the southernmost, Lecheyacu. The first 

 has in it an island rock, and (which is very decisive) is 

 surrounded by rolled pumice. Each of these lakes 

 marks the commencement of a valley ; the two valleys 

 unite, and their broader prolongation bears the name of 

 the Volcan de Ansango ; from the margins of the two 

 lakes there proceed narrow lines of rocky fragments 

 quite similar to those above described, which do not fill 

 up the valleys, but rise in their middle, as dikes of 200 

 and 250 French feet (213 and 266 English). A glance 

 at the topographical plan in the " Atlas geographique et 

 physique" of my American journey (pi. 26) will render 

 these relations clearer. The blocks, here also, are partly 

 sharp-cornered, and partly scoriaceous at the edges, and 

 even have a burnt appearance. The mass is black, and 

 resembles basalt with scantily interspersed glassy felspar ; 

 some fragments are a blackish brown, and have a faint 

 pitchy lustre. There is, however, an entire absence 

 of the olivine which is so abundant on the Eio Pisque, 

 and at Gruallabamba, where I saw basaltic columns 72 

 feet high, and fully 3 feet in diameter, containing both 

 olivine and hornblende interspersed. In the Ansango 

 dike many slabs, split into that form by weathering, 



