474 THE MARQUETTE IRON-BEARING DISTRICT. 



All of them are well banded, even where their substance has been entirely 

 recrystallized, and most of tliem are schistose. 



In thin section the crystallized tuffs are seen to be made up of a 

 crystalline aggregate of quartz, cloudy feldspar, small greenish-yellow 

 biotite flakes, occasional anhedra of green hornblende, and little irregular 

 grains of magnetite. The biotite is so arranged as to give a foliated sti'uc- 

 ture to the section, while at the same time it is more abundant in certain 

 bands than elsewhere, thus producing the banding noticed in the hand 

 specimen. In the midst of this groundmass there are often embedded 

 broken fragments of an altered plagioclase, or even entire crystals of this 

 mineral. (PI. XXXII, fig. 3.) These rocks still show evidence of their tuffa- 

 ceous chai'acter, though some of them contain small quantities of sediments. 

 Their decomposition, recrystallization, and the changes that have been 

 effected in them by dynamic processes have effaced most of the marks of 

 their original nature, but here and there, where a larger fragment or a com- 

 plete crystal of plagioclase has resisted alteration, the marks of tuffaceous 

 origin are still clearly legible. Most of the quartz and of the biotite in the 

 rocks was derived most probably from a strongly feldspathic, tuffaceous 

 dust, though a small quantity of the former mineral may originally have 

 been present as a sediment. 



In the nonschistose tuffs the tuffaceous structure is too evident to be 

 mistaken. In these rocks large anhedra of green amphibole are scattered 

 through a matrix which is composed of broken crystals of plagioclase in a 

 groundmass of smaller fragments of the same mineral, little wisps of brown 

 biotite, nests of chlorite and calcite, and a still finer matrix of the same 

 substances, cemented together by a mass of crypto-crystalline quartz. The 

 biotite seems to have come from chlorite on the one hand and from plagio- 

 clase on the other. The chlorite, in turn, appears to have come from a 

 basic glass, or possibly from augite, since it often contains within its mass 

 "divergent radial" plagioclase microlites. 



In addition to the fragments of feldspar mentioned above, there may 

 be noticed in a few specinens an occasional fragment that resembles a piece 

 of an altered glassy rock. These fragments now consist of little flakes of 

 muscovite, a very few of biotite, and grains and crystals of magnetite, all 



