CLASSIFICATION OF ORE DEPOSITS. 59 



Derby, Amer. Jour. Sci. 9 III. xxxvii., p. 109.) The action of surf 

 or smaller shore waves is also a favorable agent. The throw of the 

 breaker tends to cast the heavier material on the beach, where its 

 greater specific gravity may hold it stationary. The heavier min- 

 erals may be sorted out of a great amount of beach sand. Mag- 

 netite sands, which have accumulated in this way, are of quite 

 wide distribution, and at present are of some though not great 

 importance. (Example 15.) With the magnetite are found grains 

 of garnet, hornblende, augite, etc., and often ilmenite. Gold is 

 concentrated in the same way along the Pacific by the wash of 

 surf against gravel cliffs. In abandoned beaches of Lake Bonne- 

 ville, near Fish Springs, Tooele County, Utah, placers of rolled 

 boulders of argentiferous galena have been worked. 



A superficial deposit of somewhat different origin is the bed of 

 hematite fragments that mantles the flanks of Iron Mountain, Mis- 

 souri, and runs underneath the Cambro-Silurian sandstones and 

 limestones. This seems to have been formed by the subaerial decay 

 of the inclosing porphyry. The heavier specular ore has thus been 

 concentrated by its greater specific gravity and resistant powers. 

 (See R. Pumpelly, " The Secular Disintegration of Rocks," Proc* 

 Geol Soc. Amer., Vol. II., December, 1890.) 



1.06.21. There remain a few of great importance, but whose 

 geological history is less clearly understood. They are nearly all 

 involved in processes of regional metamorphism, and therefore in 

 some of the most difficult problems of the science. Lenticular beds 

 or veins of magnetite and pyrite that are interbedded with schists, 

 slates, or gneisses are the principal group. Such magnetite bodies 

 have been regarded as intruded dikes, as original bodies of bog ore 

 in sediments which have later become metamorphosed, and as. 

 concentrated delta, river, or beach magnetite sands. It is possible 

 that in instances they may be replaced bodies of limestone, after- 

 ward metamorphosed. The lenticular shape and the frequent over- 

 lapping arrangement of the feathering edges in the foot wall are 

 striking phenomena. 



The overlap was referred by H. S. Munroe in the School of 

 Mines Quarterly, Vol. III., p. 34, to stream action during mechanical 

 deposition, and a figure of some hematite lenses in the Marquette 

 region was given in illustration. The arrangement in instances 

 also suggests the shearing and buckling processes of dynamic meta- 

 morphism and disturbance. The individual lenses, now in linear 

 series, were thus all one original bed. The crumpling of the 



