Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/452

342 contained in the alluvial deposit, partly in the form of loose pieces and partly enclosed in fragments of rock. Some may also have

Fig. 4. — Valley-section, showing the Formation of "Tierra de flor" from "Moco de hierro."

1. "Cascajo," or felstone. 2. "Moco de hierro." 3. "Tierra de flor."

been derived from chalybeate waters coming from other sources than the alluvium itself. In some cases, the "moco" is of undoubted alluvial origin; and the miners, finding that where they met with a bed of "moco" they usually had a good pay-dirt, or "greda," came to the conclusion that the "moco" was a good indication of gold deposits of all sorts. I must further add that the alluvial "moco" lying above the pay-dirt has occasionally furnished nuggets covered with a black coating of oxide of iron, like some that are found in the "tierra de flor."

From all this it will be seen that there is nothing in the mode of occurrence or mineral contents of the "tierra de flor" to militate against the hypothesis that it is neither more nor less than "moco" decomposed in place, or decomposed "moco" washed down the hillsides. Should all the "moco" be of alluvial origin, we may expect a pay-dirt below it in the higher plateaux, just as we find one in the more recent deposits; and it is possible that the nuggets in the red dirt are mainly derived from a pay-dirt underlying the "moco," and not from the "moco" itself.

Some assays of "moco" that I made at Caratal gave gold; some that have been made since gave either no gold or merely traces. There are therefore two points which remain to be settled with regard to the "moco":—

(1) Whether it often contains gold enough to make it a workable deposit.

(2) Whether all "moco" is really a deposit like the "moco" of the "placer" diggings near the town, or, in other words, whether it overlies a pay-dirt or deposit of alluvial gold.

4. Recent Stream-beds. — When the dry season commences, the rivers and streams sink rapidly; many dry up entirely; others still have a few waterholes left; while the Yuruari always flows a little, except in seasons of most extraordinary drought. In any case, banks of sand and gravel are left high and dry, or partially so. On digging through them, a gold-bearing bed is often met with, which furnishes nuggets and small grains of gold on washing. The amount of work which goes on with these deposits at the present time is but small, though at first they were the only ones that were known.

I now come to the question of annual produce. Mr. Mathison,