454 



GEOLOGY 



are of finer sediments. It is thought that the gravel of the conglom- 

 erates was accumulated along ancient shores, and that as the shores 

 advanced upon the land, finer off-shore sediments were deposited 

 on the shore gravels. Conglomerate beds which are not basal also 

 occur, and point to changes in the conditions of sedimentation even 

 where unconformities were not developed. 



The Proterozoic systems contain thick and extensive beds of 

 quartzite, composed chiefly of grains of quartz, firmly cemented. 





Fig. 346. Section of the Proterozoic at a point in northern Michigan 



Archean granite; Ala (Ajibik quartzite), Als (Siamo slate), and Aln 

 (Negaunee [iron-bearing] formation) are Huronian formations. A ui 

 (Ishpeming formation) and Aum (Michigamme formation) are Ani- 

 mikean formations. Aed, eruptive diabase or diorite. Length of sec- 

 tion, 3 miles. (U. S. Geol. Surv.) 



Fig. 347. Section showing the relations of the Archean, Huronian, and 

 Animikean at one point in the Marquette (N. Mich.) region. JRqr 

 Archean granite; Ala (Ajibik quartzite), Als (Siamo slate), and Aln 

 (Negaunee [iron-bearing] formation) are Huronian formations. Aui 

 (Ishpeming formation) and Aum (Michigamme slate) are Animikean 

 formations. Length of section, 2 miles. (U. S. Geol. Surv.) 



The quartz grains probably came from granitic rocks, and their 

 separation from the other materials of the rock indicates the thor- 

 ough 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. Lime- 

 stone also is present, from which it is inferred that the sea of the 

 time had become calcareous by processes similar to those now in 

 operation (p. 290), and that a portion of the calcareous content of 



