C URRENT PRE- CA MBRIA N LIT ERA TURE 5 I 3 



Various igneous rocks intrude in an intricate manner both are 

 Upper Huronian and the Lower Huronian series. 



In the following paragraphs the descriptions of the formations we 

 summarized somewhat more in detail. 



The Archean. — The Archean consists mainly of massive and 

 schistose granites and of gneisses. Nowhere in the Archean have any 

 rocks of sedimentary origin been discovered. The Archean has been 

 cut by various igneous rocks, both basic and acid, at different epochs. 

 These occur in the form both of bosses and of dikes, the latter some- 

 times cutting, but more ordinarily showing a parallelism to, the folia- 

 tion of the schistose granites. The granites must have formed. far 

 below the surface, and therefore must have been deeply denuded 

 before the transgression of the Lower Huronian sea. The Archean 

 granites and gneisses and the earlier intrusives alike have been pro- 

 foundly metamorphosed, and at various places have been completely 

 recrystallized. 



The Lower Huronian series. — The Sturgeon quartzite, the first deposit 

 of the advancing sea, when formed consisted mainly of sandstone, but in 

 places at the base it consisted of coarse conglomerate. The conglom- 

 erate is best seen in the Sturgeon River tongue. Elsewhere evidence 

 ■of conglomeratic character at the base of the formation is seen, but the 

 metamorphism has been so great as nearly to destroy the pebbles. 

 However, in the Sturgeon River tongue is a great schistose conglom- 

 erate, which, while profoundly metamorphosed, still gives evidence of 

 the derivation of its material from the older Archean rocks. The 

 sandstone has been changed to a vitreous, largely recrystallized quartz- 

 ite, which now shows only here and there vague evidence of its clastic 

 character. 



The Sturgeon formation varies from probably more than iooo feet 

 in thickness in the Sturgeon River tongue to less than ioo feet in 

 thickness at places in the Felch Mountain range, and is altogether 

 absent in the northeastern part of the district. 



In the southeastern part of the district the Sturgeon quartzite is 

 overlain by the Randville dolomite. In the central part of the district 

 the quartzite between the Archean and the Randville is so thin that it 

 cannot be represented on the maps as a separate formation. In the 

 northeastern part of the district a quartzite, resting on the Archean, 

 but occupying a higher position stratigraphically than the Randville 

 dolomite, is overlain by an iron-bearing formation. It appears, 



