SUBSTITUTION. 399 



which they investigated. Had they been deceived as to the process of 

 depo.sition and were tlie ores in reahty deposited in interstitial spaces in 

 tlie limestDiic or from sohitions carrying Uirge qnantities of carbonates of 

 calcinni and magnesium, these would have been abundant in tlio ore. 



Alleged cases of substitution. — Tlio tlieory tluit ciunabar has been deposited l>y 

 sultstitution for rock has often been maintained. So far as I know it was 

 first suggested by de Prado, who believed that in the Almaden mine cinna- 

 bar had replaced a portion of the quartzite All the geological observers 

 who have written on this deposit since de Prado have reached or adopted 

 the same opinion, but tlie only proof given is that some of the impregna- 

 tions are so rich as to preclude the supposition that cinnabar occupies only 

 interstitial space. This is a statement whicli can be to some extent tested 

 by computation. Quartz sand, well shaken down, weighs 120 pounds per 

 cubic fool, while solid quartz weighs 165 pounds per cubic foot. The 

 packed sand therefore contains 27 3 per cent, of interstitial space. Were 

 this filled with cinnabar of a specific gravity of 9, the mass would contain 

 almost exactly 56 per cent, by weight of cinnabar, or over 48 per cent, of 

 quicksilver. No very definite idea is presented by "well shaken sand," 

 but it at least represents an accepted experimental result comparable with 

 the conditions to be expected in natural sand beds. Were the sand com- 

 posed of spherical grains all of the same size and as closely packed as 

 possible, so that every sphere was in contact with twelve others, the mass 

 would contain 26 per cent, of interstitial space,' and were tins filled with 

 cinnabar the mass would contain nearly 47 per cent, of quicksilver. The 

 richest impregnation whicli I was able to find in the Almaden mine or nt 

 the furnaces contained only 33 per cent, of metal and the average yield 

 of the mine for the past twelve years has been only 9 per cent If one 

 allows 1 per cent, for loss or assumes that the ore really contained 10 per 

 cent., the volume occupied by the cinnabar was only 3.7 per cent., which 

 is less than half of the interstitial space in some indurated sandstones em- 

 ployed for paving streets. The richness of the impregnations is thus cer- 

 tainly not such as to prove that replacement of quartz by cinnabar has 



It 



' Accurately, as I compute it, 1 — ,— = 25. 95 per cent. 



