394 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



THE AMAZON PLAINS REGION. 



It is to be regretted that so little systematized information is avail- 

 able concerning the Amazon region. It has been explored principally 

 along its great waterways, and the forest has prevented travelers from 

 obtaining comprehensive views of its physical features, which are of 

 relatively minor relief. There are some grassy plains. These are of 

 insignificant extent as compared with the tree-covered area. Most 

 of the sheets of Raimondi's map in the Amazon region are without 

 hachiires, and AVolf has called attention to the fact that the mountains 

 shown to the east of Ecuador and in a region which Raimondi did 

 not visit are wholly fanciful. A chain of hills or an escarpment gives 

 rise to the falls of the Madierra River, but further than this there is 

 little found in the writings of explorers excepting the mention of 

 bluffs along the streams and occasionally hilly areas. Accordingly, 

 the region must be for the present dismissed without further attempt 

 to describe or outline its physical features. 



Sedimentary Formations. 



cambrian. 



The Cambrian has not been identified in Peru by means of fossils. 

 In some instances in the literature the Cambrian has evidently not 

 been considered as a separate era, but has been included in the 

 Silurian according to former usage. Accordingly formations have 

 been discussed in connection with the Silurian which may be of 

 Cambrian age. Steinmann (1904) has described green slates near 

 Chanchamayo, whiclj he says are surely pre-Silurian, but the ab- 

 sence of fossils does not permit of their age being proved. He men- 

 tions "■ having lost his collections of fossils from Bolivia which 

 would have thrown light on the Cambrian and Silurian formations. 



SILURIAN. 



In his section from southern Peru into Bolivia d'Orbigny (1848) 

 described the Silurian as represented in the Cordillera Oriental, 

 where it has associated with it granite, which he stated forms the 

 axis of the mountain range and constitutes some of the highest peaks. 



Forbes (1861) outlined the area of the Silurian as extending from 

 north of Cuzco in Peru along the Cordillera Oriental into Bolivia 

 and southward to beyond Potosi. He found it to present physical 

 features similar to the Silurian of Europe. He says that it consists 



« Introduction to paper by A. Ulrich on " Palseozoische Versteinerungen aus 

 Bolivien." 



