The Or iff In of the I'laccr Gold of G'liiiuut. 209 



name this ironstone " roche a ravet " (cockroach-rock), the small 

 cavities serving well for the dwelling-places of blnWi orientalis. 



In many places M'here the laterite and concretionary ironstone have 

 been w-asherl from the slopes of the hills and re-arranged l)y the action 

 of running water, either in ravines or at the bottom of larger valleys, 

 the ironstone, together with more or less of the quartz gravel, gives 

 rise to beds of ironstone gravels and ferruginous conglomerates. 



Where the bulk of the laterite consists of ver}^ finelv divided 

 I)articles it is very absorbent of water. When saturated or nearly 

 saturated with water, in place of becoming plastic and tenacious as true 

 clavs do, it approximates in character to a viscous liquid, transmits 

 hydrostatic pressure, flows more or less freely under the influence of 

 gravitation, as on the slopes of the hills, and allows the quartz gravel 

 and the heavier minerals to sink through it. The latter action, in places, 

 results in the formation of beds of angular quartz gravel in the lower 

 jDarts of the laterite. 



During the decomposition of the l:)asic rocks the disseminated gold 

 in them enters into solution in the soil-waters, sinks with them into the 

 laterite, and is either re-deposited on small particles of the metal itself 

 adding gradually to their size, and on the surfaces and in the fissures 

 of the quartz, whether in the form of gravel or in sheets, or becomes 

 concentrated in the concretionary' injnstone. 



The concentration of the metal in the ironstone has been repeatedly 

 proved. Levat, who first pointed out the imjtortance of this concen- 

 tration, gave a series of determinations in his work, showing that in 

 various specimens of the ironstone the gold ranged in proportion from, 

 in round figures, one and a quarter pennyweights to nearly fifty 

 ])ennyweights to the ton. Du Bois stated that the ironstones contain 

 from two to six and a half pennyweights of gold to the ton. Lungwitz 

 alluded to the occasional richness of the concretionary ironstone, whilst 

 I have found in various specimens that its contents vary from traces 

 to as much as fifteen pennyweights of gold to the ton. 



Dr. Lungwitz has produced a remarkable proof of the solubility of 

 gold in the soil-waters. He showed that the ashes of trees that had 

 grown on the auriferous laterite of Omai contained small quantities of 

 gold, the proportion of gold in the ashes of the trunks of the so-called 

 " ironwood," varying from about two to ten grains of gold per ton of 

 ash. In the ash of the upjDer part of the trunks near the branches he 

 found as much as twenty-eight grains per ton. 



Du Bois, working on material grown in Surinam, was not able to 

 confirm Lungwitz's results, and suggested that trials carried out in the 

 conditions under which Dr. Lungwitz worked were not free from doubt. 



I made the following experiments to assist in elucidating the 

 (|uestion : Through the kindness of Mr. Dunn I obtained a piece, 

 some seven feet in length, of the trunk of an " ironwood " tree, grown on 

 the laterite covering the mass of auriferous aplite at Omai. This was 

 most carefully cleaned, sections were cut off from both ends and 

 rejected, and other sections were cut ofi' for examination. The selected 

 l^ieces were sepai-ated into the bark and the interior wood, the trunk 

 yielding 18 2^er cent, of the former and 82 per cent, of the latter. These 



