394 ANNUAL REPORT 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 
hachures, and Wolf 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, which 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. 
Tn 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 
4 Introduction to paper by A. Ulrich on “ Paleozoische Versteinerungen aus 
Bolivien.” 
