314 THE JOURNAL OF GEOLOGY. 



THE FORMATION OF HALOID COMPOUNDS IN ORE DEPOSITS 

 IN ARID REGIONS. 



The formation of chlorides and other haloid compounds 

 has already been mentioned as one of the phenomena of super- 

 ficial alteration in ore deposits. As soluble chlorides and some- 

 times other haloid compounds are common in surface waters, 

 chlorides and the allied compounds are not at all uncommon as 

 alteration products, especially in such cases as that of silver, 

 where the chloride, bromide and iodide are insoluble compounds, 

 and are not leached out. For this reason, chloride ores of 

 silver are found to a greater or less extent in almost all silver 

 districts in America, Europe, and elsewhere, but the occur- 

 rence of such compounds in very large quantities in certain 

 parts of North and South America deserves special explana- 

 tion. 



Over a large part of the arid region of the west, lying between 

 the Rocky Mountains and the Sierra Nevada, ores containing 

 chloride of silver (cerargyrite) are abundant, and sometimes the 

 bromides and iodides also occur ; in fact, parts of this region are 

 characterized by chloride ores. They are especially well devel- 

 oped in parts of New Mexico, Arizona, Utah, Nevada and other 

 states and territories, and it seems probable that their abundance 

 can be traced to the effect of the peculiar climatic conditions 

 which have prevailed in that region in late geologic times. 

 Most of this arid country was once covered with numerous bodies 

 of water, some of them of great size. In late geologic times, 

 however, these began to dry up, until their waters no longer rose 

 high enough to have outlets, and then, as a natural result, they 

 became highly impregnated with salt and other saline matter. 

 Finally, they became desiccated, leaving deposits of various 

 earthy and saline materials in their old basins, and among the 

 most common of these was common salt. It seems probable 

 that the abundance of chloride ores is due to the action of 

 this salt on the pre-existing ore deposits of the region, in the 

 basins of the lakes, and that the smaller quantities of bromides 

 and iodides were formed by a similar action of the soluble 



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