Section from Illapel to Combarbala; gypseous formation with silicified wood. Panuncillo. Coquimbo; mines of Arqueros; section up valley; fossils. Guasco, fossils of. Copiapo, section up valley; Las Amolanas, silicified wood. Conglomerates, nature of former land, fossils, thickness of strata, great subsidence. Valley of Despoblado, fossils, tufaceous deposit, complicated dislocations of.
At the bottom of these sands a green clay occurs, containing a small oyster, Ostrea cyathula, Lam., which, although of slight thickness, is spread over a wide area. This clay rests immediately on the Paris gypsum, or that series of beds of gypsum and gypseous marl from which Cuvier first obtained several species of Palaeotherium and other extinct mammalia.
The saline or gypseous interstratified beds may have been produced by submarine gaseous emanations, or hot mineral springs, which often continue to flow in the same spots for ages. Beds of rock-salt are, however, more generally attributed to the evaporation of lakes or lagoons communicating at intervals with the ocean.
But this is the case; we find those fissures filled both with calcareous, gypseous, and pyritous substances.
The discovery of the remains of Palaeotherium and other mammalia in some of the upper beds of the calcaire grossier shows that these land animals began to exist before the deposition of the overlying gypseous series had commenced.
The whole pile of strata belonging to both formations is inclined, apparently from the above-mentioned axis of Pluclaro, at an angle of between 20 and 30 degrees to the east. I will here give a section of the principal beds met with in crossing the entire thickness of the gypseous strata. Firstly: above the porphyritic conglomerate formation, there is a fine- grained, red, crystalline sandstone.
The strata, where first met with, after leaving the gypseous formation, are inclined westward at an angle of only 20 degrees, which further on increases to about 45 degrees.
In this last place, according to the report of the Indians, great masses of sulphur are found, not in a gypseous or calcareous rock, but at a small depth below the soil, in a bed of clay. This singular phenomenon appears to me peculiar to America; we found it also in the kingdom of Quito, and in New Spain.
Probably pot-stone, which is employed in China in the manufacture of cheap ornaments. Gypseous refers probably only to the degree of hardness. In the Christy collection, in London, I saw a stone of this kind from the Schiffer Islands, employed in a contrivance for the purpose of protection against rats and mice.
We may safely conclude, that at a former period the valley of Tenuyan existed as an arm of the sea, about twenty-miles in width, bordered on one hand by a ridge or chain of islets of the black calcareous shales and purple sandstones of the gypseous formation; and on the other hand, by a ridge or chain of islets composed of mica-slate, white granite, and perhaps to a partial extent of red granite.