

I 



-0 



MUSEUM OF COMPARATIVE ZOOLOGY. 



21 



are regarded as sedimentary deposits from water, subsequently more or 

 less altered by beat, pressure, and cbcmical waters acting during im- 

 mense periods of time. Tbe Lake Superior-Barnum deposit evid^ently 

 has a bottom, which will be reached within a period, of which it is worth 



while for the present generation to take some heed The time 



may come when, havnig worked out the steep, upturned edges of the 

 basins, and the flatter or deeper portions of the deposit arc reached, ore 

 properties will be valued somewhat according to the number of acres 

 underlaid hj ore, as coal now is. Passing to the east portion of the Lake 

 Superior mine, I confess myself unable to give any intelligent hypothe- 

 sis of its structure There seems to have been such a gathering 



together, crumpling, squeezing, and breaking of the strata, as nearly to 



obHterate the stratification The remarkable features are the 



great masses of light grayish-green chloritic schist, having a vertical 

 east and west cleavage, no discernible bedding planes, and liolding small 

 lenticular masses of specular ore, which conform in their strike and dip 

 with this cleavage, and which seem to have no structural connection 

 with the main deposits. They appear like dykes of ore, squeezed out of 

 the parent mass, which we may suppose t*o have been in a comparatively 

 plastic state when the folding took place; or they may have been small 

 beds, contained originally in the chloritic schist, and brought to their 

 present form and position by the same causes, which produce the cleav- 

 age in the schist Thepeculiar nature of the hanging wall of the 



Lake Superior mine deserves further notice. Instead of the quartzite, 

 whicii we have hitjierto found overlying all the deposits of rich ore, we 

 have here a magnesian schist very similar to, if not identical with, that 

 already mentioned as being associated with the ore." (/. c, pp. 138 - 140.) 

 "AH the Iluronian rocks north, east, and south from the Jackson 

 mine are below, or older than the ore format Urn (XllI)., and all the 

 rocks to the westward and inaiile of the ore-basin are youncjer, hence 

 above it." (/. c, p. 143.) '*The iron-bearing or Iluronian series of rocks 

 are stratified beds, the principal ore formation being overlaid by a 

 quartzite XIV., and underlaid by a diorite, or greenstone XI. This^ire 

 formation is made up, first, of pure ore; second, of 'mixed ore' (?'. e. 

 banded jasper and ore); and third, a soft, greenish schistose, or slaiy 

 rock (magnesian), wliich occurs in lens-shaped beds which alternate witli 

 ore, tlms often dividing the formation into two or more beds of ore. sepa- 

 rated by rock. Usually the beds of both ore and rock thin out as they 

 arc followed in the direction of the strike from a centre of maximum 

 thicluiess, producing irregular, lentiform masses. Since their ori-inal 



