Geology of the Andes. 565 



Manseriche. The limestone at the Pongo yielded me a 

 Protocardia, a linguiform Ostrea, and an Exogyra of Cre- 

 taceous type. All the Pongos on the Upper Maranon are 

 made through limestone mountains. The Punta de Schal- 

 ca is also a prolongation of the calcareous range which 

 crosses the Huallaga at the Pongo de Aguirre. It is prob- 

 able, therefore, that this western wall at the head of the 

 Amazons valley is of Cretaceous age. 



The Cerro de Icuto is flanked on the east with salifer- 

 ous red sandstone. It contains the valuable salt-mines of 

 Cachipuerto, on the Cachiyacu; and without doubt the salt- 

 hills of Chasuta and Pilluana on the Huallaga belong to the 

 same formation, as also the gypsum-beds in the elevated 

 ridge separating the Huallaga from the Ucayali. The Cerro 

 de Sal, farther south, near the head of the Pachitea, may 

 likewise be contemporaneous.'^ The Icuto rock is unfos- 

 siliferous, and I could not find its relation to the Schalca 

 limestone. The great Moyobamba valley, inclosed be- 

 tween the Schalca and Piscoguanuna ranges, is lined with 

 friable shales of divers colors — red, yellow, purple, blue, 

 and black — with overlying soft, white sandstone. Drs. Rai- 

 mondi and Spruce refer this to the Triassic. Near Tarapoto, 

 where the shales contain Ammonites of immense size, there 

 are jointed columns of trap-rock and cliffs of white salt. 



In crossing the Andes in the latitude of Lake Titicaca 

 eastward, we first find Oolitic formations largely covered 

 with intrusive rocks. After passing the summit of the 

 coast Cordillera, we have purely sedimentary strata, con- 

 torted but dipping easterly — conglomerate, sandstone, slate, 

 and Jurassic limestone. Then follow, in succession, Trias- 

 sic beds (remarkably like those in the Moyobamba valley, 

 capped with white sandstone, and broken by protruding 



* The gorge of Tunkini on the Upper Ucayali is described by Castelnan as 

 "freestone." 



