HURONIAN FORMATIONS 



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or their metamorphic equivalents. Before being cemented or 

 otherwise solidified into firm rock, their materials were gravel, sand, 

 mud, etc. The manner in which such materials are derived from 

 older formations and transported to places of deposition, has been 

 explained in earlier chapters. 



Basal conglomerate is of common occurrence at the bases of the 

 several systems of the Proterozoic. There are also conglomerate 



Fig. 300. Section of the Proterozoic at a point in northern Michigan. (^?), 

 Archean granite. The other formations are Proterozoic. Length of section, 3 

 mik-s. (U. S. Geol. Surv.) 



Fig. 301. Section showing the complex structure of the Archean and Proterozoic 

 formations at one point in the Marquette (N. Mich.) region. & gr, Archean gran- 

 itV. The other formations are Proterozoic. Length of section, 2 miles. (U. S. 

 Geol. Surv.) 



beds which are not basal, and they point to changes in the condi- 

 tions of sedimentation even where unconformities were not de- 

 veloped. Quartzite, composed chiefly of grains of quartz firmly 

 cemented, occurs in thick and extensive beds. The quartz grains 

 probably came from granitic rocks, and their separation from the 

 other materials indicates the thorough decomposition of the rock, 

 and ample opportunity for the rolling and rounding of the grains 

 before they came to rest. As the quartzites of the Proterozoic are 

 thousands of feet thick in some places, great bodies of rock must 

 have been decomposed to furnish so much sand. There are also 

 great beds of shales, or their metamorphic equivalents, which are 

 interpreted as the clayey products of the decomposition which set 

 the quartz free. Limestone is present, from which it is inferred that 

 the sea had become calcareous by processes similar to those now in 

 operation, and that a portion of the calcareous content of the 

 waters was extracted and deposited. 



The inference that these ancient sediments were deposited in 

 the same manner as sediments of modern times is supported by 



