42 /. D. Whitney on the Ores of Iron in the Azoic System. 



oxyd disseminated through their mass, as one of their essential 

 ingredients, that they might almost be called ores. These erup- 

 tive masses include the largest and purest deposits of ore which 

 are known in the Lake Superior or the Missouri iron regions ; 

 but there are other localities in both these districts where the 

 mode of occurrence of the ore is somewhat different, and where 

 the evidences of a direct igneous origin are less marked. This 

 class comprehends those lenticular masses of ore which are usu- 

 ally included within gneissoidal rocks, and whose dip and strike 

 coincide with that of the gneiss itself, but whose dimensions are 

 limited. Such is the character of most of the Swedish deposits, 

 and of many of those of Northern New York. Such beds of ore 

 as these may in some cases be the result of segregating action ; 

 but the facts seem rather to indicate that they are made up of 

 the ruins of preexisting igneous masses, which have been broken 

 and worn down, during the turbulent action which we may 

 suppose to have been preeminently manifested during the azoic 

 epoch, and then swept away by currents, and deposited in the 

 depressions of the sedimentary strata in process of formation. 

 In confirmation of this hypothesis in regard to the origin of 

 these lenticular masses of ore in the gneissoidal rocks, it may 

 be noticed that the ores occurring in this form and position are 

 less pure than those of decidedly igneous origin, as if they had 

 become more or less mixed with sand during the process of re- 

 construction, so that they not unfrequently require to be separated 

 from their earthy impurities by washing before they can be ad- 

 vantageously used. Again, it may be observed in the case of 

 some of the ore-beds of this class, that the bed-rock or foot-wall 

 is considerably rougher or more irregular in its outline than the 

 hanging wall or roof, as if depositions had taken place upon a 

 surface originally rough and uneven, the upper surface of the 

 ore being considerably smoother and more regular than the lower 

 one, and sometimes separated from the rock above by a thin 

 seam of calcareous matter. 



There is still another form of deposit which is not unfre- 

 quently met with in the Lake Superior region, and which may 

 be seen on a grand scale in the Pilot Knob of Missouri. This 

 consists of a series of quartzose beds of great thickness, and 

 passing gradually into specular iron, which frequently forms 

 bands of nearly pure ore, alternating with bands of quartz more 

 or less mixed with the same substance. Some of the deposits in 

 the Lake Superior region are of this class, and they are very 

 extensive, and capable of furnishing a vast amount of ore, al- 

 though most of it is so mixed with silicious matter, as to require 

 separating by washing, before use. Heavy beds of nearly pure 

 ore occur at the Pilot Knob, interstratifled with beds of a poorer 

 quality. Deposits of this character are usually very distinctly 



