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NOTES OF A BOTANIST
Below the sandstone (which is repeated lower down) there are
alternating layers of pudding-stone and shell-marl, the former
consisting of rounded pebbles united into a compact but sometimes
fragile mass by an argillaceous cement. The pebbles are
nearly always egg-shaped, often the size of an ordinary hen's egg,
and might seem water-worn, until being broken across they are
found to consist of concentric coatings, varying in their mineral
constituents but all more or less ferruginous. . . .
The shell-marl, or shell-rock as it might more properly be
called, is one mass of fragmentary crushed fossil molluscs, chiefly
bivalves and cirripeds, welded together by a tenacious ochry
cement, from which they are often with difficulty separated even
by the hammer. Rarely do both molluscs and cement yield to
the action of water. . . .
Beneath all these strata, which are so nearly horizontal that
there has plainly been no great convulsion since they were
deposited and they are at least 200 feet thick there is a bed of
compact argillaceous shales, which are tilted up at a considerable
angle. At Payta, where this deposit is of immense thickness and
apparently forms the great mass of the mountain called the Silla,
it puts on the appearance of slate, being of a dull dark blue
colour, and almost as hard as primary slate ; but at Amotape
what is evidently the same formation is usually of a greyish colour,
and much more easily broken.
Returning to the surface the plateau or tablazo the most
remarkable feature is the quantity of white sea-sand that is
accumulated and driven about by the winds in many parts of it.
The whole country, however, is by no means covered with sandhills,
as one might suppose from some accounts that have been
given of it. The great accumulation is in depressions and hollows
towards the northern and eastern sides of the desert, whither it
has been borne by the prevalent southerly and south-westerly
winds. . . .
In proceeding from Payta northwards towards the valley of the
Chira, we find the tablazo strewed with fragments of filteringstone,
clay-stones, etc., but we come on no sand until nearing the
valley of the Chira, or even in some places (where the cliff is
steep) until descending into the valley itself. We then find the
cliff faced with sloping ridges of sand, blown over it by the wind,
sometimes reaching into the river itself, whose waters are
continually carrying off portions of them towards the sea. It is
curious to see old Algarrobo trees with merely their heads out of
the sand, but still growing and verdant ; while others, entirely
suffocated, show no more than a few dead twigs above it. These
enormous ridged heaps are found all along the southern side of
the valley, but nowhere pass the river to northward, for the sand