SAN LORENZO. 597 
its course is indicated by an imperfect ravine or ditch. This cliff 
of shale extends north for a few hundred feet, with an even altitude, 
and then there is another fault, beyond which the beds dip eighteen 
degrees to the southwest-by-west. 
Passing farther northward, just to the southeast of the north peak, 
the rocks dip sixteen degrees to the southwest-by-south ; and the layers 
of the peak itself dip ten to eighteen degrees to the southwestward. 
Of the faults represented in figure 1, the most northern exhibits 
a dislocation of twenty feet, the rocks on the south side having sub- 
sided: the fissure formed has been filled by the comminuted material 
of the adjoining layers. In the other fault, the layers on the south 
side have been raised one foot. 
If we may infer the total number of faults in San Lorenzo, from 
those observed in the northern third of the island, it cannot be less 
than thirty ; and if this be no exaggeration, we have evidence that this 
small ridge of rock has been rent into thirty pieces or more. All the 
parts still remain together, only a little displaced. Yet on the south, 
there is an islet called the Fronton, with a few intervening rocks, which 
appear to be disjoined fragments from the main ridge. As a general 
rule, the rocks on the north side of the faults have been most elevated 
by the displacement. The period of this fracturing remains undeter- 
mined. The only facts bearing upon the subject are, first, the smooth 
surface of the island, showing that sufficient time has since elapsed to 
smooth over its broken features; and second, the calcareous cement 
which unites the broken fragments in the fissures, are additional evi- 
dence that the rocks were under water for some time after the faults 
were made. 
Fosstls.—Fossils have been observed in the San Lorenzo rock by 
different travellers.* In the few rambles by the writer, three species 
were detected. Two of them, a Turbo and a Trigonia, occur in a 
single layer of the gray sandstone, one to two feet thick, at different 
places on the coast fronting northeast, just above high water. ‘The 
third species, a large compressed Nautilus, was found under the 
northern peak of the island among the loose stones on the shore. From 
the species, it is quite probable that the deposits should be referred to 
the Oolitic era. The unbroken condition of the bivalves renders it 
altogether probable that the rock containing them, an argillaceous 
sandstone, was originally the bed in which they lived. 
* See American Journal of Science, xxxviil. 201, 202. 
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