238 THE ANDES OF SOUTHERN PERU 



in narrow "pongos" or canyons filled with huge bowlders and 

 dangerous rapids. The hills become mountains, ledges appear, 

 and even the heavy forest cover fails to smooth out the natural 

 ruggedness of the landscape. 



It is only upon their eastern border that the Silurian series 

 includes calcareous beds, and all of these lie within a few thou- 

 sand yards of the contact with the Carboniferous limestones and 

 shales. At first they are thin paper-like layers; nearer the top 

 they are a few inches wide and finally attain a thickness of ten 

 or twelve feet. The available limestone outcrops were rigor- 

 ously examined for fossils but none were found, although they 

 are lavishly distributed throughout the younger Carboniferous 

 beds just above them. It is also remarkable that though the 

 Silurian age of these beds is reasonably inferred they are not 

 separated from the Carboniferous by an unconformity, at least we 

 could find none in this locality. The later beds disconformably 

 overlie the earlier beds, although the sharp differences in lithology 

 and fossils make it easy to locate the line of separation. The 

 limestone beds of the Silurian series are extremely compact and 

 unfossiliferous. At least in this region those of Carboniferous 

 age are friable and the fossils varied and abundant. The Silurian 

 beds are everywhere strongly inclined and throughout the eastern 

 half or third of their outcrop in the Urubamba Valley they are 

 nearly vertical. 



In view of the enormous thickness of the repeated layers of 

 shale and sandstone this series is of great interest. Added im- 

 portance attaches to their occurrence in a long belt from the 

 eastern edge of the Bolivian highlands northward through Peru 

 and possibly farther. From the fact that their disturbance has 

 been on broad lines over wide areas with extreme metamorphism, 

 they are to be separated from the older mica-schists and the 

 crumpled chlorite schists of Puquiura and Pasaje. Further rea- 

 sons for this distinction lie in their lithologic difference and, to 

 a more important degree, in the strong unconformity between the 

 Carboniferous and the schists in contrast to the disconformable 

 relations shown between the Carboniferous and Silurian fifty 



