86 H. E. Gregory — A Geologic Reconnaissance 



a separate heading. 28 * Post-Pleistoceue or Recent formations 

 include gravel and sand plains resulting from the erosion of 

 glacial gravels and the late readjustment of stream channels 

 as the result of natural and artificial processes. Travertine is 

 found along nearly all drainage channels crossing the Yucay 

 limestone. At many waterfalls and rapids the lime of the 

 highly charged water is deposited on reeds and various 

 grasses, preserving their forms in great detail. On the west 

 branch of Rio Sebolla-Huaycco, about half a mile above its 

 mouth, the stream is bordered by a cliff 40 feet in height com- 

 posed entirely of lime-incrusted plants of species identical with 

 those which constitute the harsh, stunted vegetation of the 

 present day. At other places casts of plants ground up and 

 dissolved by running water are redeposited as beds of banded 

 gray travertine containing minute fragments of leaves, branches, 

 and fibers. 



IV. THE IGNEOUS ROCKS. 



Introduction. 



The igneous origin of two rock masses in the Cuzco Valley, 

 El Rodadero and Rumiccolca, has long been recognized, but 

 the indiscriminate use of the terms "amphibolic rock," "basaltic 

 rock," "trachyte," "granite," "sienite," and "porphyry" ren- 

 ders most of the published descriptions of little use for geologic 

 purposes. Microscopic analyses by Duehas 29 of specimens 

 from El Rodadero and from Huaccoto mark the beginning of 

 systematic petrographic studies in this part of Peru. During 

 the course of the field season of 1912 most of the exposures of 

 igneous rock within the Cuzco Valley were visited, and it is 

 believed that the geologic map combined with the petrographic 

 notes gives a fairly complete description of their distribution 

 and character. 



Extrusive masses are typically represented by the volcanoes 

 at Huaccoto and Rumiccolca ; intrusives by El Rodadero and 

 the outcrops at Chchiraura ; and clastic volcanic materials may 

 be studied on the slopes of Cerro Pachatucsa. 



Volcanic activity in the Cuzco Valley has been local and 



relatively feeble. The exposures of igneous rock, both intrusive 



and extrusive, are small, the largest (Rumiccolca) covering 



an area of less than 2 square miles. Except at Atasccasa the 



original masses have suffered little from erosion. 



- s a See p. 39. 



29 Op. cit., pp. 178, 192, 1907. 



