416 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



some of the white chalk material and some beds of impure concre- 

 tionary limestones similar to what occurred at the type locality near 

 Pisco, The so-called chalk material was analyzed by the Corps of 

 Engineers of Mines and found to consist principally of silica, with 

 small amounts of lime and alumina. A microscopic examination 

 showed it to contain many diatoms and what appeared to be vol- 

 canic ash. 



In traveling by steamer from Pisco to Lomas the Pisco formation 

 can be seen forming the sea cliffs and rising to the table land of lea. 



Although some fossils have been found, they have not been studied 

 critically. The age of the Pisco formation is not surely known. The 

 writer has assigned it to the Pliocene provisionally, since it is over- 

 lain by deposits which are probably of Pleistocene age, and there is 

 no information which shows the necessity of assigning it to an 

 earlier time. 



TERTIARY OF THE SOUTHERN COASTAL PLAINS. 



The Moquegua formation. 



The writer has given this name to the formation which occupies 

 the southern coastal plains. It has been described locally, by Forbes 

 and others, as already mentioned in this paper, but no one had 

 journeyed sufficiently over the plains to learn that it was coextensive 

 with them. The strata which constitute it can be studied conven- 

 iently in the valley of the Moquegua Kiver, especially near the town 

 of the same name. It is also well exposed in the valleys of all the 

 streams which cross the plains, since they have cut deep canyons. 

 The eastern limit of the formation is at the foothills of the Andes, 

 and the western limit is formed by the chain of coast hills. It 

 reaches to the Pacific Ocean in the interval between the coast hills 

 of Peru and the Morro of Arica, which is the northern extremity of 

 the coast hills of Chile. The character of the rocks which constitute 

 the Moquegua formation has been well outlined by Forbes. They 

 consist of sands with some clays, a large quantity of detrital material 

 derived from igneous rocks, but especially noticeable are the thick 

 beds of volcanic material which appear to have been deposited in 

 water and interbedded with sands. In the valley of the Kiver Vitor, 

 which descends from the Andes past the volcano Misti which is 

 located near Ariquipa, beds of lava may be seen which have de- 

 scended from the volcano and extended over the plains, where they 

 form a capping on the Moquegua formation. The age of the volcanic 

 rocks is not certainly known, and there has been no opportunity to 

 determine the age of the Moquegua formation, since no fossils have 

 been found. It is generally stated that the volcanoes of southern 



