REVIEWS 525 



rocks are exposed west of the mountains, where rapid erosion has cut 

 through the breccia fan and tablazo deposits. 



The Tertiary rocks are chiefly shales, with subordinate sandstones: 

 the Zorritos formation, 5,000 feet thick and more, Miocene; the Lobitos 

 formation, 5,000 feet plus, and the Negritos formation, 7,000 feet plus, 

 both Eocene. They are of shallow-water origin, and their deposition, 

 in view of their great thickness, was probably accomplished by sub- 

 sidence of the sea bottom at intervals. These rocks are not folded; but 

 close faulting has broken them into blocks, tilted in different directions, 

 and at angles of 5° to 35°. This has been done by Andean upwarpings, 

 in such a manner that this coast strip constitutes part of a "crush belt" 

 along a geo-fault, the axis of which is believed to be in the Pacific Ocean 

 at the edge of the continental shelf. 



After this faulting there was a long period of erosion, with later sub- 

 mergence. In Quaternary times, a series of nearly vertical movements 

 took place in which the Andes Mountains acted as a hinge, the free edge 

 being the Pacific fault. There was a series of four episodes, beginning in 

 each case with a marine transgression, the cutting of an extensive marine 

 erosion plane, and the deposition of marine beds upon it; and terminating 

 with an uplift, which elevated a plateau-like sea floor, or tablazo. The 

 oldest and highest of these tablazos now stands at an elevation of 1,100 

 feet above tide, the ancient cliff line being only one or two miles from the 

 foot of the Andes. The lowest and latest is now in the phase of 

 emergence, giving partially flooded coastal plains. 



The tablazo deposits consists of loosely compacted shell limestone, 

 with large pebbles locally. This peculiar type of sediment is explained 

 by the fact that no streams enter the ocean for 140 miles along the coast, 

 except during the floods which occur twice or three times in a century; 

 the accumulating calcareous material being free from elastics except at 

 such times. 



After the uplift of the first of these tablazos, exposing a large flat 

 surface at the foot of the mountains, the floods from the latter aggraded 

 great piedmont alluvial "breccia fans" on the tablazo; and the subse- 

 quent cutting back of streams into the latter gave terraces in the alluvium. 

 With the development of the succeeding tablazos, similar aggradation 

 and subsequent terrace cutting occurred in connection with each. 



The author draws some interesting conclusions from his field observa- 

 tions as to the length of time, concluding that not one ten thousandth 

 part of Quaternary history could have taken place within the last 500 

 years. This conclusion is based upon the hundreds of feet of observed 



