GEOLOGICAL EXPLORATIONS AND LITEEATITEE. 63 



yet l)e<'n discovered in it. It is liighly nuigtiesiaii, and is a ddlouiite rather than a 

 limestone proper. It is also impure, from the in'esencc of siliceous and aluminous 

 material. 



Detrital beds. — Overlying this formation at some points is n bed of white gran- 

 ular quartzite, which indicates that the deposition of calcareous sediment was followed 

 by an accumulation of quartz sand. 



Upon this lie beds of quartz-schist and argillaceous mica-schist, having together 

 a thickness of about 400 feet. These were probably originally a deposit of sand and 

 sandy, calcareous, and magnesian clay, derived mainly by ordinary wear and decom- 

 position from the adjacent land. 



Above these is a thick series of beds of iron-bearing and siliceous scliists and 

 quartzites, which now form the crest of Penokeo Iron range. These have together a 

 known thickness of about 800 feet. They appear to have consisted originally of beds 

 of fine impure sand, with lenticular layers of iron ore thickly sandwiched through 

 the mass. 



OrigiH of the iron ore. — The origin of the siliceous material can be confidently 

 refenejl to the atmospheric decomposition and the wearing and assorting work of 

 streams and waves acting upon the granitic and other siliceous rock of the adjacent 

 Laurentian land. To account for the iron ore is less easy. It occurs (1) iu thin 

 layers, or (2) more frequently in lenticular masses a few inches in thickness inserted 

 irregularly among the laminations of the schist, and (3) in scattered particles dissem- 

 inated through the rock. In its present tbrm it is largely magnetic ore, though the 

 specular variety is present. Ri some places both theee forms have been reduced to 

 hematite and limonite by subsequent changes. 



The manner in Mhich the iron is associated with quartzose material bears a 

 somewhat close resemblance to the way iu which magnetic-iron sands are distributed 

 through the quartz sand of certain beaches, as may be seen at many points on the 

 shore of lake Michigan ' at the present time, and as is reported to be the case on the 

 coast of Labrador, where the ocean is now acting upon the same formation that the 

 ancient Huronian sea did iu its day in the Penokee region. This similarity .suggests 

 a like derivation — an exi)laiiation applicable to many of the features of the deposit — 

 but it does not very satisfactorily account for other characteristics. It certainly 

 seems inapplicable to some of the great iron deposits that occur in the Huronian 

 series. 



The most probable explanation of the massive iron-ore beds in general refers 

 their origin to organic agencies. Meteoric waters charged with decomposable 

 organic matter, i)ercolating through the soil and surface rock, change its iron ingre- 

 dient from the insoluble to the soluble form and bear it onward, and at length out 

 into some adjacent body of water, into which the drainage is discharged. Here it is 



I Vol. II, p. 239. 



