PERU. 599 
CONCLUDING REMARKS ON THE GEOLOGY OF PERU. 
The occurrence of gypsum in connexion with sandstones upon the 
Andes has long been known; and Dr. Pickering observed large 
quantities of it at Bafios in the Chancay Valley, fifteen thousand 
feet in elevation, which he was informed had been brought from Casa 
Cancha, fifteen miles to the southward, along the summit of the moun- 
tain. Sandstone and shale resembling that of San Lorenzo were also 
met with; but the sandstone was in part a conglomerate, or contained 
small pebbles. About two leagues southwest of Casa Cancha, an 
Ammonite was found by Dr. Pickering, imbedded in an argillaceous 
rock, which is described in the Appendix. 
From the pebbles of the Callao beach, it appears that the same 
porphyritic sand-rocks and conglomerates occur here as in the Chilian 
Andes. Many of them so resemble porphyry in colour and disseminated 
feldspar crystals, that it is with the greatest difficulty they can be dis- 
tinguished. Usually some imbedded pebble may be detected, though 
often faintly, and this is the only evidence of a conglomerate or com- 
posite character. They afford an example of a rock made of porphyry 
earth and pebbles, altered by heat so as to imitate very exactly a true 
porphyry. 
Effects of an Earthquake-—Many instances of a rotation of the 
blocks of columns by earthquakes are on record. Yet the subject is 
one of sufficient interest to authorize this allusion to a fine example of 
it in Lima, represented in the sketch here given. This is but one 
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among many examples ; for Lima is properly a city of ruins, owing to 
the earthquakes it has experienced. Walking through the city, we 
observe scarcely a spire which has not thus lost something of its 
