332 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 some- 
times 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 l^ed 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 sand- 
hills, 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 northw^ards towards the valley of the 
Chira, we find the tablazo strewed with fragments of filtering- 
stone, 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 
