of the Cuzco Valley, Peru. 87 



The rocks are prevailing basic in composition. Specimens 

 assigned to the syenite family were collected at two localities ; 

 varieties of andesite, basalt, or diabase constitute the other 

 outcrops. The generalization of Dueiias applied to the entire 

 Department of Cuzco is thus confirmed : "El tipo mas general 

 es el de andesitas con horneblenda 6 bien con aujita 6 todavia 

 con mica negra, presentandose a menuclo el caso de tenerse 

 andesitas que ademas de un piroxeno 6 anlibol, encierran 

 tambien biotita. 30 



The lavas at all localities present substantially the same 

 appearance and are believed to represent simultaneous outflows. 

 The rock is but slightly weathered, and the flows retain to a 

 large degree their original topographic expression. They ante- 

 date the last period of glaciation, however; the edges of the 

 igneous mass at Ichchu-Orcco have been scored by ice, and 

 till is banked about the base of the Huaccoto volcano. On the 

 basis of these observations the most recent expression of vol- 

 canism is tentatively assigned to late Tertiary time. The age 

 of the intrusive rocks is likewise unknown. The igneous mass 

 underlying the fortress of Sacsahuaman penetrates Upper ( ?)• 

 Cretaceous sediments; most of the other dikes and bosses are 

 surrounded by strata of earlier age. It is possible that some 

 of the intrusive rocks are contemporaneous with the beds in 

 which they are now found, a view supported by the presence 

 of ash and tuff in the Pachatucsa and Huayllabamba forma- 

 tions. Detailed field work is necessary before the age relations 

 of the intrusive igneous rocks can be discussed with confidence. 



All the igneous masses are in a belt with northwest trend 

 on the north side of the Cuzco Valley, lying between Huatanay 

 and Urubamba rivers, no igneous rocks being found south of 

 the valley axis. When the zone containing the volcanic rocks 

 is extended southeastward it embraces the isolated volcano of 

 Tinta, the hot springs of Agua Caliente, and the great faults 

 bounding the sunken block at Lake Titicaca. Innumerable 

 small faults are also included. It may be that these features 

 indicate a zone of crustal weakness associated with the Tertiary 

 uplift of the Andes. 



Description - of Areas. 



The outcrops of igneous rock shown on the geologic map 

 were examined in the field and a suite of specimens was col- 

 lected. In the microscopic study of the rocks valuable 



30 Op. cit., p. 31. 



