8 R. A. F. PENROSE, JR. 



over it, and frequent deposits of saline materials. Through these 

 loose materials frequently protrude isolated hills and knolls of strati- 

 fied and igneous rocks similar in nature to many of the rocks found 

 in the adjoining mountain ranges; and doubtless rocks of hke 

 character underlie the loose sediments of the pampa. The surface 

 materials of the pampa are probably of post-Tertiary age and repre- 

 sent an old sea bottom formed by the deposition of sediments in a 

 now extinct inland sea, or a series of lakes, which once extended 

 from the Andes to the Coast Range, over the whole extent of what is 

 now the pampa. 



The Coast Range bordering the pampa on the west is composed 

 of a variety of rocks varying greatly in different places from Tarapaca 

 southward through Antofagasta and Atacama. Old crystalline 

 rocks, including gneisses, granites, etc., frequently occur. Stratified 

 rocks of probably both Paleozoic and Mesozoic ages, including 

 sandstones, limestones, and shales, are abundant in many places, and 

 are much folded, contorted, and broken. They are intersected in 

 many places by igneous intrusions. As we cross the pampa and 

 approach the Andes, both stratified and igneous rocks again appear 

 through the loose sediments; and in the high Andes immense areas 

 of late volcanic flows are found. 



MODE OF OCCURRENCE OF THE NITRATE DEPOSITS IN THE 

 TARAPACA REGION 



The pampa region has a general slope from east to west, that is, 

 from the foot of the Andes to the foot of the Coast Range, though it is 

 so gradual as to be often unnoticeable to the eye. As a result of 

 this slope the lowest part of the pampa is along its western border, 

 where it abuts against the Coast Range foothills. It is along this zone 

 that the nitrate deposits occur, and in the province of Tarapaca they 

 occupy a narrow north-and-south belt following this position for over 

 one hundred miles. The surface of the pampa is here almost always 

 impregnated with more or less saline matter, which sometimes 

 becomes so abundant as to form beds several or many feet in thick- 

 ness. These are practically superficial deposits, though they are 

 sometimes capped by earthy materials for some feet in depth. Their 

 surface, when exposed, often presents a rough and more or less 



