Page:The Journal of geology (IA journalofgeology21894univers).pdf/330

The formation of chlorides and other haloid compounds has already been mentioned as one of the phenomena of superficial alteration in ore deposits. As soluble chlorides and sometimes 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 occurrence of such compounds in very large quantities in certain parts of North and South America deserves special explanation.

Over a large part of the arid region of the west, lying between the Rocky Mountains and the Sierra Nevada, ores containing chlorides 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 developed 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