340 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9,
formation obtained from Dr. Plassard, who was at Caratal while
this stuff was being worked.
Fig. 3. — Section of Alluvial Digging near Nueva Providencia.
The following is the succession of the beds:—
1. Soil.
2. Red clay, showing no signs of stratification.
3. Soft clayey "moco de hierro."
4. Hard moco de hierro," brown iron ore, with pieces of quartz in it and a little clay.
5. Blocks of vein-quartz, often auriferous.
6. "Greda," or pay-dirt, a yellow ferruginous clay containing nuggets and small grains of gold.
7. "Cascajo," decomposed schist, forming the "bed-rock."
About half a mile from the town, on the road to Tupuquen, are the "Planada" diggings, said to have been worked with much profit in 1857. The section of one of the Planada pits is as follows:—
1. Soil.
2. Clayey gravel.
3. Blocks of quartz.
4. "Greda," or pay-dirt.
5. "Cascajo," the bed-rock.
In a neighbouring pit I saw about 6 feet of clay and then 6 feet of gravel, the bottom being filled with rubbish. The pebbles found in the gravel are mainly composed of the greenish grey felstone, known as "piedra negra; "subangular and rounded pieces of ferruginous hornstone are also common. Higher up the valleys the placer diggings are shallower.
3. Tierra de Flor and Moco de Hierro. — In many places in the Caratal district, just below the soil, at a depth of 18 inches to 3 feet,
