part 1] THROUGH THE ANDES OF PEKU AND BOLIVIA. 27 



Many of the hornblende-andesites bear a striking resemblance to 

 the trachy-andesites described by me from Mount Taapaca in 

 Northern Chile, while the augite-andesites are very similar to those- 

 from Mount Tacora. 



The volcanic peaks, with the exception of Ubinas a little farther 

 south, which was smoking at the time of our visit, are no longer- 

 active, though El Misti still preserves a remarkabby fine crater, and 

 signs of latent activity are not wanting in the district. 



The effects of a small volcanic earthquake which occurred on 

 April 28th, 1911, were described to me by Mr. Vogel, of Arequipa. 

 While looking down from the astronomical observatory he noticed 

 a thick cloud of dust passing like a sudden wave through the city, 

 and at the same time felt a distinct earthquake shock. He after- 

 wards found that the dust was due to the collapse of buildings- 

 constructed of ' adobe ' and light volcanic tuff. The wave had left 

 a well-defined track in its wake, on either side of which the houses- 

 remained unaffected. Such an extremely-localized shock suggests- 

 the formation of a subterranean fissure or dyke. 



Some distance north-east of El Misti, in a little tributary valley 

 of the River Sumbay, we also encountered numerous hot springs 

 highly charged with sulphuretted hydrogen and alkaline salts. 



The general sequence of the volcanic deposits is well exposed in 

 the valley of the River Chili, the railway-cuttings round the foot 

 of Mount Chachani, and in the gorge of Uyupampa beyond Yura. 

 Here, too, they consist chiefly of false-bedded, white, yellow, and 

 grey tuffs, with occasional darker bands of coarse ash. 



When the line is followed towards the east, the country is found 

 to be very like that of the Mauri-River district in Bolivia, for it is 

 buried beneath a thick mantle of pumiceous tuffs which give the 

 same peculiarly monotonous aspect to the landscape. In many 

 localities, however, the surface is relieved by fine examples of 

 ' Zeugen ' or rock-tables, formed of hard blocks of lava capping 

 pillars of less resistant tuff which have been undercut by the 

 erosive action of wind and rain. In places these are so numerous 

 that they are termed by the natives ' rock-forests.' The tuffs fre- 

 quently exhibit polygonal jointing on a very minute scale. 



In addition to this volcanic covering, the structure of the under- 

 lying country is further obscured by extensive swamps and thick 

 alluvial gravels, composed chiefly of pebbles of quartzite and rocks 

 of igneous origin ; these sometimes cover Avide tracts, such as that 

 between Vincocaya and Crucero Alto, and have the appearance of 

 being old lake-deposits. The quartzite-pebbles have evidently 

 been derived from a series of beds which only occasionally crop out 

 at the surface. 



Rocks of this nature were first met with on the banks of a small 

 tributary of the River Sumbay, where they were found to consist 

 of unfossiliferous white quartzites, dipping 10° south-westwards. 

 They are broken through by a small volcanic neck filled with a 

 coarse agglomerate. Similar beds of quartzite also crop out on the 

 road to Pucacancha, and occasionally form low hills bounding the 

 alluvial flats between this point and the summit of the line. 



