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There are certain broad divisions in North America of which particular types of deposits are characteristic. These divisions in general may be assigned to different geologic epochs. Several prominent mineral belts are well known. Gold quartz veins in schistose rocks are characteristic of a zone stretching for a long distance through California and to the north and south. A series of lenticular copper deposits occurs along the foot-hills of the Sierra Nevada in California.

Vhere mineralization has proceeded contemporaneously with the deposition of the enclosing bed the resulting deposit is known as a seam, or bedded deposit; in this class are also included those replacements of similar occurrence the origin of whose mineralization is not apparent. The criteria for distinguishing between bedded deposits and intercalated veins are given in a preceding paragraph. Beck makes a further distinction between interbedded deposits, which are overlain by other strata, and superficial deposits where there are no overlying beds, as, for example, beds of bog iron ore.

While not a numerically important type, mineralized beds of conglomerate form the ores of two of the most important mining districts in the world. The origin of these deposits is not clear, and while to a certain extent similar, they posses features that render difl&cult their classification with the more common and better understood deposits.

An important class of oredeposits is that in which the valuable minerals occur as minute particles, or narrow seamlets, or stringers, throughout a large mass of enclosing country rock. The number of such mineralizations whose primary ore is of payable grade is probably small, but these deposits, especially those that contain copper, are of the greatest importance where enriched by secondary processes.

A majority of veins are in part, at least, the result of the replacement of their walls by mineralizing solutions, and in many cases the process of fissure filling was probably so subordinate as to be practically negligable; the original fissures of replacement veins, which were probably narrow, acted chiefly as channels for the passage of the replacing and mineralizing solutions.

The filling of open fissures or preexisting cavities in rocks is a process of great importance in ore deposition; while many veins are due to this process alone, a majority of mineral veins are probably the result of both replacement and the filling of open spaces. Open spaces are caused by irregular fissuring accompanied by a moderate movement, sufficient to bring projection opposite projection and concavity opposite concavity, and thus cause pinches and swells in the fissure and resulting vein.

Deposits that were formed in remote geological ages are likely to have been deeply buried, and to have undergone rearrangement under the stresses of dynamo-regional metamorphism. The recrystallization result-ing from these processes may so completely change the enclosing rocks to crystalline schists as to hide completely their original character, it being difficult to distinguish schists that result from the metamorphism of sedimentary beds from those that were originally igneous rocks.

The characteristic minerals of pegmatitic deposits* are magnetite, bornite, arsenopyrite, molybdenite, cassiterite and wolframite, associated with quartz, muscovite, alkali feldspars, tourmaline, apatite, fluorite, spodumene, and more rarely, hornblende and soda-lime feldspars. These deposits, which are of deep-seated origin, contain little gold and silver and except where associated with stockworks and cassiterite-bearing impregnations they are irregular in value and of slight economic importance, except for mica and minerals of the rare earths.

Contact deposits are deposits formed along the contacts between intnisives and their enclosing rocks, or in these rocks in immediate proximity to the intnisives. They are the result of direct emanation of mineral-bearing solutions from the intruding magma. Contact deposits are usually limited to rocks that exert strong precipitative action.

During the solidification of magmas under conditions that do not permit the escape of their metallic content, there is a tendency for like particles to form segregations probably through mass action, or the mutual attraction of like particles. Magmatic segregations are commonly made up of compounds of relatively low mobility.