230 THE MARQUETTE IROX BEAIIING DISTRICT. 



sliattcring- the rock en masse, the chistie character of the orig-hial grains is 

 usually still iiiaiked, and they are easily discriminated from the secondary 

 cherty quartz. In other phases of the rock the stresses were relieved by 

 movement affecting the mineral particles. The original quartz and feldspar 

 grains were granulated, and the latter were decomposed. Secondary quartz 

 formed both in the interstices and in veins, and sericite developed. This 

 process of secondary silicitication and development of sericite seems to be 

 in direct ratio to tlie severity of the mechanical movement affecting the 

 individual grains. Between the phases in Avliich the relief is largely by 

 brecciation and those in which it is largely by mashing there are all 

 gnidations, an intermediate phase showing the partial granulation of the 

 fragmental grains, their cementation by silica, and at the same time numer- 

 ous ^•eins of secondary cherty quartz. As has been said, the extreme 

 idteration of the original quartzose sandstone resulted in peculiar, vitreous, 

 cherty-looking quartz-rocks, and that of the original feldspathic debris 

 resulted in a sericite-schist. The facts that the sandstones became cherty 

 brecciated rocks and that the coarse and line muds became schists are 

 probably explained by the brittle character of the first and the plastic char- 

 acter of the second, one yielding mainly b}' fracture, the other mainly 

 by flow.' 



DELATIONS TO UNDEELYING FORMATION. 



The fact that basal conglomerates are found at variovis places near the 

 contact of the Mesnard ([uartzite and the Basement Complex has already 

 been mentioned, and the localities at which these conglomerates occur have 

 been mentioned. These contacts are of such character as to indicate that 

 the Mesnard quartzite is separated from the Basement Complex by a great 

 unconformity. Since in these basal conglomerates are numerous pebbles 

 and bowlders of granites, gneisses, and schists from the Basement Complex, 

 the major part of the comi)lex history of the Archean was comj)lete 

 before the Mesnard (piartzite was deposited. Erosion had before this 

 time cut so deeply into it as to bring to the surface in some places coarse- 

 grained granites and in other places the truncated, foliated layers of the 



' Principles of North American jire-Canibrian gculogy, by C. K. Van Rise: Sixteenth Auu. Kept. 

 U. S. Geol. Survey, Part 1, 189G, pp 601-603. 



