PETEOGRAPHIGAL CHAKACTER OF GOODRICH QUARTZITE. 413 



as has beeu before explained, .secondary magnetite formed. The detrital 

 micaceous hematite is usually easily discriminated from the crystal-outlined 

 secondary magnetite. While a considerable percentage of the iron oxide 

 of the ore was present as detritus, in no case does it appear that the 

 material was rich enough for merchantable ore before the secondary con- 

 centration, and often the secondary magnetite and its alteration jJi'oducit, 

 martite, are the predominant constituents of the ore. 



Microscopical. — With tlic uiicroscope, the basal conglomerate resting on 

 the Ajil)ik quartzite is found to have a background consisting of quartz 

 grains set in a more or less abuiidant sericitic, cherty, and iron-impregnated 

 matrix This matrix may be so abundant as to separate the fragmental 

 grains, or may be sparse. In this background are found complex fragments 

 of quartzite the individual grains of which are rounded, and fragments of 

 sericite-slate and sericite-schist, all identical with these rocks in the Ajibik 

 quartzite. 



In the Republic trough, where the Goodrich quartzite in part rests 

 directly upon the Archeau, the schist-conglomerate found at the bottom of 

 the detrital formation has as a matrix a micaceous quartz-schist. In certain 

 varieties feldspar is abundant in the background, and it becomes a mica- 

 gneiss. In this background are oval or ribbon-like areas of quartz or of 

 feldspar, which represent the mashed pebbles of the conglomerate. Occa- 

 sionally these pebbles contain both quartz and feldspar, and represent 

 complex fragments derived from the granite. The quartz is always and 

 the feldspar is usually shattered, and along the crevices of the feldspar 

 mica and quartz have developed. Frequently the residual feldspar and 

 the secondary quartz and mica form an interlocking mass. Were it not 

 for the pebble-like areas these rocks would be regarded as completely 

 crystalline schists. 



Where the Goodrich quartzite rests upon the Negaunee formation 

 there are three main jihases of material: (1) Chert and jasj^er conglomerate, 

 (2) recomposed jasper, and (3) ore. 



The chert and jasper conglomerate may have a sparse or an abvmdant 

 matrix. In the first case the matrix consists of small, simple, fragmental 

 grains of quartz, complex particles of ferruginous chert and jasper, and 

 iron oxide. In passing to the less strongly conglomeratic phases the matrix 



