REVIEWS 635 



The zinc ores are regarded as having been deposited by under- 

 ground waters. Emphasis is laid on their accumulation along 

 synclinal troughs and water-way breccias. "The details of the theory 

 of the accumulation of the Arkansas ores along synclines and other 

 water-ways were first suggested by field observations made in this 

 region in 1889, and the whole theory has been much strengthened by 

 subsequent work." 



According to their genetic relations there are three kinds of sul- 

 phide ores : {a) the bedded deposits, which are contemporaneous 

 with the rocks in which they occur ; (/;) the veins and other fracture 

 deposits in which the ores are of later age than the accompanying 

 beds, and (c^ the breccia deposits not formed on fractures, but like- 

 wise of later age than the accompanying beds. In addition to the 

 sulphide ores there are carbonate and silicate ores, derived by altera- 

 tion from the sulphides and forming genetically a fourth class. 



Regarding the origin of the bedded deposits, it is stated that they 

 "have originated for the most part where we now find them." The 

 cherts are made of silica of organic origin, that is, they were deposited 

 over the sea bottom as silicious skeletons of diatoms or other micro- 

 scopic remains of plants or animals. The zinc came from the adjacent 

 land areas of the period in which these beds were laid down. Upon 

 entering the sea the zinc-bearing waters had their zinc contents pre- 

 cipitated in the form of sphalerite or zinc sulphide by the organic 

 matter that contributed the silica of the chert beds. The zinc crystal- 

 lized out while these silicious sediments were yet soft and yielding. 

 In time the sediments hardened and formed the firm, flinty rocks and 

 pressed closely about the zinc blende crystals. ' 



"The crystals of zinc blende, however, were not originally as large 

 as we now find them in the disseminated ores, even where these crys- 

 tals are no larger than a pin head. They were at first even micro- 

 scopic, but, as Ostwald has pointed out, there is a tendency in such 

 cases for the small crystals to pass into solution and to recrystallize 

 upon the larger ones which grew at the expense of the small ones. 

 In the bedded deposits this took place before the enclosing sediments 

 were hardened." 



The vein deposits are those occupying the spaces left by fractures 

 in the strata. The ores are confined to the fractured zone and to its 

 immediate walls. When the ore is found in the walls it seldom pene- 

 trates them to any considerable depth, but is confined to small 



