OCTOBEE 11, 1901.] 



SCIENCE. 



577 



with this' as the primary object will discover 

 similar evidence of submerged shingle beaches 

 at many other points around the island. 



Wm. E. Rittbr. 



Univeesity of California, 

 Sept. 7, 1901. 



ZONE OF MAXIMUM RICHNESS IN ORE BODIES. 



For a long time, and among many mining 

 people, the theory has prevailed that ore de- 

 posits have been derived from the interior of 

 the earth, the mineral materials being carried 

 upward to the surface by means of heated 

 solutions. As a result, a maxim has been es- 

 tablished that ore bodies necessarily get richer 

 as depth increases. The fact that many excep- 

 tions have been found to this rule is ascribed to 

 peculiar local conditions. 



Aside from the bare statement of the general 

 rule, no limitations have been formulated by 

 the mining men. It has remained for the 

 geologists to reach measurable results regarding 

 the relative richness of ore bodies at varying 

 depths. The results are not only very satis- 

 factory, but they are totally at variance with 

 the commonly assumed formulae. Late investi- 

 gations demonstrate, both theoretically and 

 practically, that the problem has been wholly 

 misunderstood by miners ; and that the so- 

 called empirical rule has very decided limi- 

 tations. 



Contrary to opinions heretofore generally 

 held, many, if not most, ore bodies are believed 

 not to be formed by the materials coming up in 

 a superheated condition from great depths to 

 the surface of the earth. Revolutionary as it 

 may seem to many who have not followed care- 

 fully the trend of recent investigation, it appears 

 to be a fact, nevertheless, that ore bodies are to 

 be regarded as deposits formed very near the 

 surface of the earth's crust; or, to be more 

 precise, formed only in that thin outer part of 

 the zone of the lithosphere which geologists are 

 pleased to call the zone of fracture. Unusual 

 richness which many ore deposits show at very 

 shallow depths has come to be looked upon as 

 due to local enrichment long after the first con- 

 centration has taken place. 



Careful study of important ore bodies indi' 



cates that after a certain depth is reached, there 

 is frequently a very marked decrease in th^ 

 amount of ore material, until finally in some 

 cases the ores become too|lean to work. From 

 the point of view of origin, diminution in rich- 

 ness with depth is not, then, to be regarded as 

 an actual depreciatiou in grade of the ore. The 

 real status of the case is that the original depo- 

 sition of the ore has^in the upper zone under- 

 gone a greater or less augmentation in metallic 

 content since the ore bodies first began to 

 form. 



As distinct processes, the rival theories of 

 ascending solutions, descending solutions and 

 laterally moving solutions no longer find counte- 

 nance among those who have given the subject 

 of ore genesis most attention, and especially 

 among those who have'approached the subject 

 from the geological side. Ore deposition may 

 take place through all three means, which may 

 have equal importance. After an ore deposit 

 has once formed under special geological con- 

 ditions, the secondary enrichment which it may 

 undergo is believed to take place largely under 

 the influence of the"?descending solutions. 

 Therefore, in the exploitation of ore bodies, 

 everything goes to show how vitally important 

 is a full consideration of the geological struc- 

 tures presented, both at the time of the first 

 concentration and as subsequently assumed. 



Under the title of ' Enrichment of Mineral 

 Veins by Later Metallic Sulphides,' in the re- 

 cently issued Volume XL of the Bulletin of the 

 Geological Society of America, Mr, W. H. 

 Weed gives the results of his investigations 

 concerning the zones of maximum richness in 

 ore bodies. Briefly stated, the attempt is made 

 to prove : (1) that the leaching of a relatively 

 lean primary ore, commonly by surface waters, 

 will supply the material in solution for such en- 

 richment; (2) that the unaltered sulphides, es- 

 pecially pyrite, will induce precipitation, that 

 the material precipitated is crystalline, and 

 that a number of mineral species are commonly 

 formed, and are now forming, in veins by such 

 reactions ; and (3) that such minerals deposited 

 in quantity may form ore bodies of considerable 

 size (bonanzas), or may be disseminated through 

 the lean primary ore in strings and patches, 

 thus enriching the ore body as a whole and 



