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PKOCEEDINGS OF THE GEOXOGICAL SOCIETT. [June 9, 



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 dejlor^^ 

 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 un- 

 doubted 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 in- 

 dication 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 hill- 

 sides. 



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-heds. — When the dry season commences, the 

 rivers and streams sink rapidly ; many dry up entirely ; others stUl 

 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, 



