THE KEPUBLIO THOUGH. 527 



Of gneisses, properly so called, none liave been found in tlio Kepul)lic 

 area except those that have nnniistakably been derived from the normal 

 granite by dynamic metamori)hism. These are best seen in the immediate 

 neighborhood of the contacts with the overlying series, and tliey are so 

 characteristic of these contacts that where gneissic foliation is present the 

 contact may confidently be looked for close at hand. This gneissic structure 

 is largely due to the development of mica, usually inuscovite, along surfaces 

 of breaking which, while individually irregular and waving, yet in the 

 aggregate are distinctly parallel in .strike and dip to the contact surface and 

 to the bedding planes in the overlying sediments. 



The gneissic structure is most strongly developed at the contact. In 

 departing from the contact it diminishes by degrees, and finally, at dis- 

 tances which usually do not exceed 200 feet, it disappears altogether or is 

 found only in narrow, irregular, and discontinuous zones. That this struc- 

 ture is really due to the processes of dynamic metamorphism acting on the 

 normal granite is evident both from observation in the field, where it may 

 be seen in all .stages of development, and also in thin sections, where it is 

 clearly proved to result from granulation of the original quartz and felds})ai', 

 and the passing over, in some cases pai'tial and in others complete, of the 

 latter into quartz and into the new light-colored micas, which are orientated 

 with the directions of fracture. 



In the Archean areas are found certain dark-colored hornblende-schists 

 and amphibolites. These occur usually in naiTOw bands and are exceed- 

 ingly variable in the degree of schistosity which they exhibit and in 

 crystalline character. Some, at least, are without question old dikes, orig- 

 inally diabase or diorite, in which a parallel arrangement of new minerals 

 has, with more or less completeness, been effected by dynamic metamor- 

 phism. In many cases the progress of these changes may be traced from 

 a massive crystalline interior into nearly perfectly foliated zones at the 

 walls. In other cases the schists are completely ciystalline throughout, 

 and these bear no evidence of their igneous character. In age these 

 schists doubtless vary enormously. Some have furnished pebbles to the 

 basal conglomerate of the Lower Marquette series, and these pebbles are 

 as thoroughly crystalline and schistose as any of the schist bands that can 

 now be found in the Archean areas. Others are almost certainly younger 



