THE IRON SERIES (IN PART). 75 



At the Prosser mines, near Portland, Ore., deposits of limonite 

 are found in the superficial hollows of a Tertiary basalt of the 

 Cascade range. The ore contains roots and trunks of trees, and is 

 covered by a later flow of basalt. Similar bodies of limonite re- 

 sulting from basalt are known in the German province of Hesse 

 and in Ireland. 1 



2.01.09. The limonite sand, or oolite, that forms in the Swedish 

 lakes about ten meters from the banks and in water up to ten 

 meters in depth is another variety of this type. A layer half a 

 meter and less in thickness accumulates every fifteen to thirty 

 years and is periodically dredged out. The ore precipitates first 

 as a slime that breaks up afterward into small concretions. It 

 has been thought that the formation of these and similar bodies of 

 limonite has been aided by small algae and other plants or micro- 

 scopic organisms. 2 



2.01.10. Example 2. Bodies of limonite in cavities of fer- 

 ruginous rocks, on the outcrop, or below the surface, which have 

 resulted either from the alteration of the rock in situ or from its 

 partial replacement by limonite. Residual clay, quartz, and other 

 remains of alteration usually occur with the ore. Ferruginous 

 limestones are the commonest sources of such deposits, but other 

 rocks may afford them. The deposits are not limited to any one 

 geological series, but in different parts of the country occur when- 

 ever the conditions have been favorable. Some of the ore may 

 have been brought in by subterranean circulations which have 

 leached the neighboring rocks. Considerable limonite has also re- 

 sulted from the weathering of clay ironstone nodules and black- 

 band beds in the Carboniferous system (to be mentioned later), and 

 not infrequently from the alteration of nodular masses of pyrites. 



1 B. T. Putnam, Tenth Census, Vol. XV., p. 16, on the Oregon ore. 

 Tasche, Berg.- und Hutt. Zeit., 1886, p. 209; also Wurtemberger, Neues 

 Jahrb., 1867, p. 685, on the Hessian ores. Tate and Holden, "On the Iron 

 Ore Associated with the Basalt of Northeastern Ireland," Quar. Jour. 

 Geol Sci , XXVI. 151. 



2 F. M. Stapff, Zeitschr. d. d. geolog. Gesellsch., 1866, Vol. XVIII., p. 8, 

 on the geology of the ores. Sjogrun, Berg.- und Huett. Zeit., 1865, p. 116, 

 on the agency of algas. On the general formation of bog ores the fol- 

 lowing papers are of interest: G. J. Brush and C. S. Rodman, "Ob- 

 servation on the Native Hydrates of Iron," Amer. Jour. Sci., ii., XLIV. 

 219; J. S. Newberry, School of Mines Quarterly, November, 1880; J. Roth, 



Chem. und Phys. Geologie, I., pp. 58, 97, 221 ; F. Senft, Humus, Marsch> 



Tor/- und Limonit-bildungen. 



