Recent Formations. 103 



The concretionary ironstone is usually auriferous, and samples 

 which I have examined have varied in their contents from mere traces 

 of the metal to as much as sixteen pennyweig'hts per ton of the rock 

 ($14-28 value per ton of 2,000 lbs.). 



Readers who are desirous of a fuller account and discussion of the 

 Guianan laterites and concretionary ironstone are referred to the 

 monograph by G. C. de Bois, entitled "Beitrag zur Kenntnis der 

 Surinamischon Laterit un Schutzrindenbildungen," in " Tschmaks 

 Mineralogische und petrographische Mitteilungen, XXII. Band, 1. 

 Heft. 1903." 



Among the heavier minerals which tend to sink into the lower 

 layers of the laterite, one — ilmenite — is very conspicuous in the stream 

 and river gravels and sands in the laterite districts. This gives 

 rise to black sands, which usually consist almost wholly of ilmenite, 

 although in places it is mixed with more or less magnetite. The 

 sands, as a rule, contain higher proportions of ilmenite and lower 

 ones of magnetite than do the titaniferous iron-ores of the rocks 

 from which the sands have been derived, the magnetite offering less 

 resistance to deoxidising agencies than does the ilmenite. As would be 

 expected the ilmenite sands are generally accompanied by gold, partly 

 in the free state, and partly included in the mineral. Samples collected 

 by myself have been examined for their contents of precious metals, 

 with the following results, given in grains per ton (2,240 lbs.). 



Sand from Ichaura Rapids, Potaro River ... 



Sand from Kobanatuk Rapids, Potaro River 



Sand from Orimetuk Creek, near the Kaicteur, Potaro River 



Sand from near Wariri, Cuyuni River 



Ilmenite sand from a placer claim in the Konawaruk district was 

 found to be accompanied by seven pennyweights of gold to the ton of 

 the sand ; whilst sand of which the following is an analysis, obtained 

 from the valley of the Minnehaha Creek, Konawaruk River, yielded at 

 the rate of twenty-one pennyweights of gold per ton : — 



Silica 2-54 



Titanium oxide ... ... ... ... ... ... 52-03 



Iron pero.xide... ... ... ... ... ... ... 5'y6 



Iron protoxide ... ... ... ... ... ... 37'14 



Manganese oxide ... ... ... ... ... ... trace 



Ahimiua ... ... ... ... ... ... 1-87 



Magnesium oxide ... ... ... ... ... ... 0'60 



100-14 



The samples collected from near Wariri, when examined with 

 the aid of a magnifying glass, did not show any signs of free gold. 



Dr. Emil E. Lungwitz, in a paper published in 1900 in The 

 Mining Journal, Raihvay and Commercial Gazette, entitled " The 

 Placers of British Guiana," directed attention to the movements of 

 the residuary deposits when saturated with water, and showed that 



