590 PERU. 
The nature of the wood here buried has not been determined. Dr. 
C. Pickering suggested to me that it might be the recent willow 
(Salix Humboldtiana) so common in wet places in Western Peru: but 
this is a mere conjecture. 
The upper layer of this cliff, which is from four to five feet thick, 
contains large numbers of the recent shells of the coast, and the same 
are strewed abundantly over the plain above. The species are the 
common shells of the present beach, as already described by Darwin. 
They are but little altered in appearance, and some are still covered 
with the epidermis; yet they are generally brittle and many of them 
may be crumbled in the fingers. Numbers of them have the ap- 
pearance of having been much worn by the action of the sea on a 
beach. 
This layer also contains brick and tiles or pottery-ware, in small 
fragments, and in great abundance. With them I found also the jaw 
of a dog. 
On the island of San Lorenzo, there appear to be no remains of the 
recent formation which has been described, except we include as 
such, beds of shells near the shores, eighty or eighty-five feet above 
the sea. These shell beds, described with characteristic accuracy 
and detail by Darwin, le in the sandy soil instead of forming a dis- 
tinct layer, and contain the same shells as observed in the Callao 
cliff. I collected twenty-two species, several of which differ from those 
procured by Darwin. A Crepidula was by far the most abundant 
shell, as well here as in the cliffs alluded to. 
A few shells were found scattered over the hills of San Lorenzo 
nearly to their summits, which might have been driven there by the 
winds, or carried by birds. 
Towards the northeast point of the island, along the shores, there 
is a stratified bank of sand and pebbles with fragments of the slate 
rock of the island; it is situated at the head of the present beach, 
about fifteen inches above high water mark. ‘The height of the bed 
is seven feet. The lower layer consists of rounded stones, two to four 
inches in diameter, with numerous shells similar to those of the 
shores; the next above is a layer of sand with pebbles and small frag- 
ments of slate, about four feet thick; then follows a layer of coarse 
gravel, thickly abounding in shells of the species Purpura chocolata, 
with a few other species, the Purpura being the principal shell: they 
have the worn character of beach specimens. The upper layer of 
this bank is composed of fine earth, from the decomposition of the 
