PETKOGEAPIIICAL CHAKACTEK OF MESXAIU) QUAKTZITE. 225 



bowlders are well rouudeil The}- eoinprise coarse-g-raiued niuscovite-grauite 

 and i)eculiar fine-o-rained granites. The green-schist pebbles have a very 

 wide variety, including decomposed granular greenstones and various clilo- 

 ritic schists. Every phase of the basic and acid fragments is matched by 

 rocks of the Northern Complex. A comparison of the fragments with the 

 adjacent rocks of the Archean can leave no doubt that the major part of 

 the detritus of the conglomerate was derived from this source. The sparse 

 matrix between the pebbles is composed of well-rounded to subangular 

 o-rains of feldspar, of quartz, and of the finer complex detritus of the various 

 materials of the Northern Complex. A few complex cherty fragments are 

 seen. In this matrix the various feldspars are especially abundant. This 

 finer detritus is set in a still finer background of the same materials, wnth 

 much chlorite and fine secondary quartz. 



(2) The conglomerates adjacent to the Southern Complex have two 

 pliases — those that are coarse and distinctly show a conglomeratic char- 

 acter, and those that are composed of finer detritus. The latter in some 

 cases so closely resemble granite in the field that they are with difticulty 

 discriminated from it. 



The coarser phases have as predominant pebbles coarse granite, the 

 feldspar of which is nuich kaolinized, and which may be considered a kao- 

 linic quartz-schist; large irregular areas of complex quartz, wdiich may have 

 been derived from a very coarse grained granite, or may have come from a 

 quartz-schist ; and complex pebbles of altered, fine-grained biotite-gneiss. As 

 the conglomerate becomes finer-grained the complex fragments decrease in 

 quantity and are replaced by large simple fragments of quartz and feldspar. 

 Where the pebbles disappear the rock in hand specimen simulates granite 

 or gneiss. In addition to the predominant pebbles of the conglomerate, 

 there are present large complex fragments of ferruginous schist, of chert 

 or jasper, and of a quartzite-Hke rock. The ferruginous schist i)ebbles have 

 a very fiuely crystalline, quartzitic, and kaolinic background, through which 

 iron oxide is scattered or concentrated in irregular connecting layers. These 

 appear to be ferruginated, decomposed schistose rocks rather than true chert 

 or jasper. The ferruginous chert or jasper pebbles are very similar to those 

 of the Negaunee formation, but they show less banding, and the iron oxide 



MON XXVIII 15 



