1G6 THE MARQUETTE IRON-BEARING DISTIUCT. 



formed by tlie iiggreg-atioii of its jjlutes interiningled ^vitli gnuns of epidote, 

 of calcite, and of great nunibers of i-utile crystals that are usually included 

 ■within tlie chlorite. As Williams has suggested, these areas may represent 

 the basic fragments of the tuffs that have been altered beyond recognition. 

 The sericite is as abnndant as the chlorite in some specimens, while in 

 others it is ])resent in small (luantity only. It appears as colorless or light- 

 green flakes that lie scattered among the other constituents, especially 

 l)etween the chlorite prisms, and as tiny spicules that penetrate the grains 

 of the mosaic matrix. Of the calcite nothing need be said, except that it 

 occurs as nests or as grains between the other minerals. In some of the 

 more compact tuffs a second carbonate is soiiietimes met with in the form 

 of small rhombohedra. The substance is more opaque than the calcite, 

 and is of a ncIIow or Aellowish-brown cohir. It is prol)al)l}- ankerite or 

 ferruginous dolomite 



The matrix in which all the compcments of the groundmass lie resem- 

 bles A-erA" cl()scl\- the silicihed background of many aporhyolites. Under 

 Tery high powers of the microsco})e it is seen to be made up of intricately 

 interlocking, colorless grains. Their aggregate polarizes like a tine-grained 

 (piartz mosaic. No twinning bars were seen in any of the grains examined, 

 lint nuun- grains exhibit an undulatory extinction. This groundmass is 

 therefore I'egarded as (piartzose. Some of the quartz uiay lun'e been derived 

 from the original constituents of the tuff by alteration, but most of it, in 

 all i)robability, has been introduced from without. The i-ocks have evidently 

 Ijeen silicitied, for we tind ledges cut through and through l)y quartz veins, 

 ami in the microscoj)ic section these veins can l)e followed as they break 

 n\) into smaller and smaller ones, until their ramitieatious are finally lost 

 in the mosaic above mentioned. 



The evidenc-es oi' pressure resulting in mashing are not so abundant in 

 thesi' schists as tlwy are in some of the other rocks of the district, it may 

 be for the reason tliat maiiv of its effects are obscnrcil by the secondary 

 substances produced subsequent to its action. Many of the larger feld- 

 siiar particles are broken and their fragments are displaced laterally; the 

 lono-er axes of all the fragments are often approximately in the same plane; 



