34 BULLETIN OF THE 



tion of rocks of various ages and cliaracters goes to show that all rocks, 

 especially the older, have been subject to more or less alteration. This 

 alteration is accompanied by recrystallization, which often obliterates the 

 original characters. This change appears to be produced through the 

 medium of the percolating waters, and consists rather in a chemical 

 rearrangement of the constituents of the rock amongst themselves, 

 than in the deposition of any material brought in from extraneous 

 sources. The jasper and iron ores, as well as all other rocks examined 

 microscopically from this district, have suffered this alteration to a 

 greater or less extent ; therefore it is perhaps impossible at present to 

 be sure of the original state of the iron, or how many changes have 

 taken place. 



Without objecting in any degree to the idea that the ore was origi- 

 nally magnetic, certain facts indicate that the present magnetic state 

 of the iron is in some places due to secondary causes ; i. e. the heat of 

 intrusive rocks erupted since the iron ore and jasper were in place. 

 While in general the Republic Mountain ore is hematite, exceptions 

 exist. On the northerly side of the hill a " dioi'ite " dike was seen (91, 

 92). It is found that the ore was so affected by the heat of this intru- 

 sive mass that it is magnetic adjacent to it (90), while a short distance 

 away it is the normal hematite. Numerous other localities were exam- 

 ined about the hill where these secondary intrusions occurred, with the 

 same result ; the iron ore was magnetic adjacent to the dikes, but not 

 magnetic a short distance away. As a general rule, the magnetite or 

 the hematite pseudomorphs after it (martite) are found near the "quartz- 

 itc " of Brooks in this mine. Those who examine the map of Republic 

 Mountain, prepared by him,* will observe on the northern side of his 

 "quartzite," a queer tongue of it projecting into the hematite. An ex- 

 amination of this tongue at different places shows the following facts. 

 It contains numerous rounded and irregidar fragments of the iron ore 

 in it ; these fragments occur on both edges (93, 94, 90), while the cen- 

 tre of the mass is free frovn them (95). At this point it varies from a few 

 inches to two feet in width, and it is seen to break across the lamina- 

 tion, altliongh nearly coinciding with it. At another part, shown near 

 the same pit. No. 8, this rock and its contact with the "jasper" and 

 ore were well marked. The "quartzite" (115, 118) is firmly welded to 

 the ore, and breaks across the laminte, cutting them, and sending 

 tongues into the mixed jasper and ore (116, 117, 119). The junc- 

 tion is an eruptive (intrusive) one, and not that belonging to the con- 



* Atlas, GeoL of Mich., 1869-73, Plate VI. 



