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increased depth they become more numerous and continuous, until they predominate over the altered products, and finally, when the limit of alteration is reached, they entirely replace them. The planes of contact between an ore deposit and the country rock, that is the walls, afford, when well defined, easy passages for the downward percolation of surface waters, and therefore alteration frequently continues down along these lines for considerable distances after the limit of alteration in the main part of a deposit has been reached. Any other possible channels, such as the planes of contact of different minerals in banded deposits or the series of drusy cavities often found in the central parts of ore deposits, may act in the same way as passages for water. Hence the not infrequent abundance of alteration products, such as hydrous sesquioxide of iron, and native copper and silver, along the walls and elsewhere in certain deposits.

Classification of the products of alteration.—The products of superficial alteration may be divided into two general classes: (1) Those which occupy the same position as the materials from which they were derived, or are only slightly removed, and possess the same general environment. Thus the altered outcrops of auriferous quartz and iron pyrites, of argentiferous galena, of sulphides of copper and many other similar deposits, represent alteration-products occupying the same general position as the original sulphide ores; while the iron ore bodies of the Lake Superior region represent alteration-products changed somewhat in position from that occupied originally, but yet in the same series of rocks and sometimes with somewhat similar environment. (2) In the second class are included those deposits which have been entirely removed from their original position and redeposited under totally different environments. Thus, placer gold deposits, stream tin, most of the deposits carrying platinum and the allied metals, magnetic and chromite sand, the gravels and sands carrying precious stones, and many other similar deposits represent this class. They have been derived by the decay and erosion of veins, dikes or country rocks carrying the materials now concentrated in these fragmental deposits.