528 

had one great advantage ; the desert conditions that 
prevail expose clearly to view much more of the 
characters and structures of the rocks than meets the 
eye in more fertile regions. At the same time, he has 
spared no pains to make his work as complete as 
possible. In order that the fossils collected should 
be properly described he has. enlisted the services of 
a number of the leading paleontologists in this country 
and America, and their descriptions, figures, and con- 
clusions are given in full. 
The district described rarely exceeds a thousand 
feet in height above sea-level. It lies between the 
Pacific and a background of the Western Andes, which 
consist of pre-Tertiary rocks folded under the stresses 
of the zone of compression that encircles the greatest 
NATURE 

[APRIL 21, 1923 
in the immediate neighbourhood of the shore, raised 
above its previous level. It was at the same time 
broken up into numerous blocks separated by minor 
faults, often of considerable throw, constituting a 
kind of fault breccia on a gigantiesscale. 
In this aggregate of dislocated sediments the action 
of the sea, assisted by subaerial agencies, excavated 
a broad, nearly horizontal shelf, which reached to the 
foot of the mountains, and became submerged suffi- 
ciently to allow of a new series of deposits being laid 
down upon it, which must be referred to the Quaternary 
Period, as they contain remains of forms identical 
with those living in the adjoining sea. Then a period 
of elevation supervened, and the former sea-floor was 
exposed to view as a nearly level plateau—a tablaso, 

Fic. 1.—Ridge-and-furrow topography, produced where the strike of the beds is in the same direction as the prevalent wind. 
From ‘ 
of our oceans. The latest strata involved in the folds 
are of Cretaceous age, and it must have been after 
their deposition that these mountains were raised up 
and exposed to the destructive activities of sun, wind, 
and running water, and in all probability frost and 
ice as well. From the debris a great succession of 
sedimentary strata of Tertiary age were accumulated 
to a thickness of some 20,000 feet on the slowly sub- 
siding ocean floor, not without important breaks in 
the succession, for only the Eocene and Miocene are 
represented. 
The denudation of the mountains and the accumula- 
tion of the sediments over a broad tract on the margin 
of the Pacific appear to have destroyed the isostatic 
balance that had previously existed and created a 
state of strain which finally resulted in a great fracture 
off the coast, the western side being thrown down 
deep below the surface of the sea, and the eastern, 
which included such of the Tertiary deposits as were 
NO. 2790, VOL. 111] 

‘Geology of the Tertiary and Quaternary Periods in the North-west Part of Peru.” 
as it is called locally—and it still remains in many 
places almost in the same condition as it was when 
the sea left it. Its western margin was now attacked 
by the waves and a second shelf was carved out, which 
was covered by another series of deposits, and after- 
wards raised to form a second fablazo. Still another 
tablazo, possibly more, would seem to have come into 
The last tracts to be 
raised from the sea were the salina or salt plains, 
which are scarcely above the reach of the spring tide. 
Indeed, some parts are occasionally submerged. 
Remains attributed to the Incas are found upon them, 
and some of the land a few hundred yards from the 
high-water line has been irrigated, apparently by them. 
It would seem therefore that there can have been no 
appreciable change of level since the coming of the 
Spaniards. The author infers that not a ten-thousandth 
part of the Quaternary history can have elapsed in 
the last five hundred years. This would give Quatern- 
existence in the same manner. 
