SAN LORENZO. 593 
brick and tile and pottery-ware, south of Callao, might lead us to infer 
that some native settlement had there been devastated. The occur- 
rence of the shells on San Lorenzo in an irregular bed might thus be 
better accounted for than by the slow action of a beach. Yet we do 
not feel ready, without farther examination, to attribute the effects to 
this cause. Mr. Darwin infers that the shells and tile south of Callao 
were mingled together by this means, but that the land was probably 
at a lower level (eighty-five feet), to allow of the effect being produced. 
The stratified beds, forming the cliff of the shores, south of Callao 
and the plains back, leave us no room for doubting that an elevation 
has at some time taken place, of at least fifty or sixty feet, since these 
beds were deposited probably over a low region which was either per- 
manently or occasionally submerged. But to fix the time when the 
change of level took place, may require some farther attention than 
the facts have yet received. 
SECONDARY ROCKS OF SAN LORENZO. 
The island of San Lorenzo, about seven miles off the shore of 
Callao, is a narrow, elevated ridge, five miles long in a northwest 
and southeast direction, and averaging one mile in breadth. On the 
side fronting the ocean, the cliffs are abrupt, and a hundred and fifty 
feet or more in height. On the opposite side, they are from fifty to 
ninety feet in altitude, and alternate with low beaches; and above 
these cliffs, as already stated, there is a narrow and more or less 
uneven plain at the foot of the southeastern declivities of the ridge. 
The northern summit of the island was found, by the barometrical 
measurements of the officers of the EXxpedition, to be twelve hundred 
and eighty-four feet in elevation, the middle nine hundred and twenty 
feet, and the southern eight hundred and ninety-six feet. 
The rocks are either sandstone or argillaceous shale, and they are 
various in shades of colour, and frequent and abrupt in their alterna- 
tions. ‘Their decomposition has produced the thin coating of arid soil 
which covers the declivities. 
The stratification is neatly distinct in the cliffs, and may be de- 
tected over the sandy slope above, in an occasional horizontal dark 
line of rock. The colours of the shales vary from a deep red, through 
grayish, greenish, purplish and bluish shades, to a dark blue-black : 
and these several colours often succeed one another in the course of a 
149 
