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 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 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 northwards 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 



