598 PERU. 
Minerals.—Gypsum, common salt, calc spar, quartz, pyrites, and 
oxyds of iron, occur in the San Lorenzo rocks; and besides these, coal 
is reputed to form thin layers on the south side. The salt forms thin 
seams in the shale and sandstone, and the gypsum occurs both in 
seams and crystallized in fissures. At the shale cliff of eighty feet, 
already alluded to, the gypsum intersects the rock in every direction, 
and so numerous are the intersections and flexures, that the shale is 
cut up by the seams into small angular pieces. The shale often drops 
out on exposure, leaving the plates of gypsum projecting, and the 
surface of the cliff in this case appears honeycombed. 
These seams or plates are usually less than an eighth of an inch 
thick, though occasionally half an inch, and sometimes an inch. The 
thinner are fibrous, while the wider often contain handsome geodes of 
crystals. ‘These crystals are from a third of an inch to an inch in 
length, and are brilliant in lustre and perfect in form. Some of these 
geodes contain several square inches of surface covered with fine 
crystals, and would be an ornament to any cabinet. The rock is, 
however, so brittle that great care is required to preserve them from 
injury. 
The calc spar and quartz form occasional veins in the rock. Iron 
is abundant as a colouring ingredient, and occasionally the magnetic 
and red oxyds occur in narrow irregular seams. ‘The observatory 
established on the island for the Expedition, was afterwards removed 
to the main land, because the needles were affected by the iron of the 
rocks. 
The facts relating to the occurrence of gypsum in the buried wood 
and clay of the recent shore-deposits south of Callao, afford us an 
explanation of the circumstances on San Lorenzo. ‘The gypsum has 
been formed in cracks which previously intersected the rock, and 
probably by the evaporation of sea-water. 
Extent of the Formation.—The extent of the San Lorenzo forma- 
tion, or its relation to the rocks of the Andes, has not been made out. 
Immediately beyond Lima, the ridges consist of granite and syenite ; 
and according to Dr. Pickering, the same rocks prevail to a great 
extent over the forty miles to the eastward. ‘The San Lorenzo sand- 
stone constitutes a point twelve miles south of Callao; and the island 
of Pachicamac, twenty-four miles south, has the same constitution. 
Specimens from the latter place, consisting of red and gray sandstones, 
and red, gray, whitish, and dark-blue argillaceous shales, were pro- 
cured by Lieutenant Knox of the Expedition. 
