496 THE MARQUETTE lEON-BEAKlNG DISTEICT. 



under the microscope. In most of the rocks, however, nt) traces of augite 

 remam. Hornblende or chlorite has taken its place, and the rocks are now 

 either uralitic diabases, in which the hornblende is compact and pseudomor- 

 phic after the aug'ite, or ei)idiorites, in which the amphibole is iibrous, and 

 in which the ophitic structure of the diabase has in some cases entirely 

 disappeared. The rocks of both these varieties are dark-green in color, 

 fibrous in texture, and often schistose in structure. 



Under the microscope they are found to be composed most largely of 

 light-green amphibole, remnants of altered plagioclase, large plates of epi- 

 dote, much chlorite, large masses of very beautiful leucoxene surrounding 

 ilmenite, and nests of calcite. In the freshest of the uralitic diabases the 

 augite has given rise to compact, dark-g-reen uralite, and the plagioclase to 

 epidote and calcite. In the epidiorites the amphibole has become filn-ous. 

 Not only are areas formerly occupied by the augite now filled with fibrous 

 amphibole, but long, slender needles of the mineral extend far out into the 

 surrounding rock materials. 



The plagioclase in these rocks has suff"ered extreme alteration. Its 

 twinning bars have nearly disappeared and its material has been changed 

 to kaolin, chlorite, saussurite, epidote, and calcite. Sometimes one and 

 sometimes the other of these substances predominates, and not infrequently 

 there occur scattered through the slides perfect calcite pseudomorphs of 

 lath-shaped crystals of plagioclase that preserve even the twinning of the 

 original feldspar. 



The epidote is in the usual green jilates. While often an alteration 

 product of the plagioclase, it is sometimes a result of the decomposition of 

 the augite, when it is intermingled more or less freely with chlorite and 

 calcite. 'The chlorite is in little nests scattered between the other minerals, 

 and in grou[)s of fibers pseudomorphing feldspar. It is an alteration of the 

 amphibole as well as of the plagioclase. 



Further alteration of the uralitic diabases and the epidiorites gives rise 

 to chloritic rocks in which chlorite has replaced the hornblende. The 

 chlorite here is in pale-green, very Aveakly doubly refracting fibers that 

 form pseudomorphs of the amphibole and preserve the o]:)hitic texture of 

 the original rock. Epidote, calcite, and leucoxene are abundant in these 



