Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/451

1869.] LE NEVE FOSTER-CARATAL GOLD-FIELD. 341 there is a layer of red earth, often containing pieces of auriferous vein-quartz, pieces of "moco de hierro" and grains of pisolitic brown haematite, the "granson" of the miners. When washed, this "tierra de flor," or "surface earth," as it is called, furnishes, in addition, decomposed crystals of iron pyrites, a black magnetic iron sand called "arenilla," and often nuggets of gold. The manner in which the gold occurs appears to be very irregular; the miner may wash "batea" after "batea" of earth and find no gold, or only the "color de oro" (i. e. merely a few fine grains of gold), and then get several nuggets in the next lot. The largest nugget yet found in the Caratal district was obtained from the "tierra de flor," close to Nueva Providencia; it weighed 15 lbs.

In the diggings at the south-south-west corner of the town I saw "tierra de flor" lying above alluvial ground.

The following is the succession:—

1. Soil.

2. "Tierra de flor," red earth, with loose stones of quartz.

3. Ferruginous clay.

4. "Greda," or pay-dirt, yellowish clay, with blocks of quartz.

5. "Cascajo," or bed-rock, decomposed schist.

From the manner in which this "tierra de flor" occurred, I at once concluded that, geologically speaking, it was a "rainwash;" but it was some time before I understood whence it was immediately derived. At last, after talking the matter over very frequently, Dr. Plassard and I arrived at the conclusion that the "tierra de flor" is nothing more or less than decomposed "moco de hierro" washed down the hillsides.

The name "moco de hierro" is given to a highly ferruginous, rock, which assumes the various forms of:—

a. Ferruginous conglomerate.

b. Ferruginous grit.

c. Ferruginous breccia.

d. Pisolitic brown iron-ore.

It always consists mainly of limonite and earthy red haematite, with pebbles or angular and subangular fragments of quartz, schist, and felstone. When it takes the form of pisolitic brown iron-ore, it consists of a number of globular concretions of limonite. This "moco" is found in loose blocks on the surface, and often forms plateaux, sometimes more than a hundred (or even two hundred) acres in extent. The edge of the plateau is generally marked by a bold rocky escarpment. Here it may be seen that the "moco" is 6, 8, or even 10 feet thick at the least; and the same thing may be observed where a stream has cut its way down through a "moco" plateau as shown in fig. 4.

In searching for quartz-lodes I came to the conclusion that the "moco de hierro" affords no evidence of lodes in the immediate vicinity; for my observations led me to infer that it is probably of alluvial origin. I should suppose that the ferruginous matter of which it consists so largely is derived from iron pyrites, which was