636 REVIEWS 



fractures that seem to be parts of the great fissures. In appearance 

 the fissure ores are not different from the bedded deposits. But they 

 are stated to have a very different origin. The ores of this class have 

 all been brought into their present position by solution, probably 

 from the Ordovician bedded deposits. 



The question of the origin of the breccia ores "has been one of 

 the most puzzling problems encountered in the zinc regions. The 

 only theory for these formations that seems tenable is that of the 

 apparently irregular masses of breccia, that is, the breccias not upon 

 fault and such like fractures, have been formed along ancient under- 

 ground water-courses." 



One of the most suggestive points brought out in this considera- 

 tion of the zinc ores is the relation of synclines to the presence of ores. 

 Dr. Branner says: "If the hypothetical history here assigned the 

 north Arkansas zinc ores is thus far correct, we are forced to conclude 

 that the geologic structure of the region is of the utmost importance 

 in the determination of the present distribution of the ores. In an 

 elevated region of approximately horizontal or very gently folded 

 sediments, the waters falling upon the ground and soaking into the 

 earth tend to seek the bottoms of the synclinal troughs. The process 

 of ore accumulation in such a region would therefore tend to carry the 

 ores into the synclines. The rocks of the zinc region, although not 

 far from horizontal, are gently folded. Wherever folds have been 

 exposed in the zinc mines the bottoms and sides of these folds have 

 been found richer in zinc than the adjacent portions of the same beds. 

 This is a rule to which I know but few exceptions. The inference 

 seems to be warranted that the synclinal troughs should be located 

 and examined for the richer zinc accumulations." 



Of exceptional interest at this time are the notes on the faults of 

 north Arkansas. For the first time in the consideration of the zinc 

 region something tangible regarding these structures and their char- 

 acter is made available. The throw of the faults, though never very 

 great, is sometimes four hundred feet or more. The character of the 

 folds found in the vicinity of the faults is shown by numerous 

 figures. 



The illustrations are unusually good. 



C. R. Keyes. 



