516 THK MARQUETTE IRON-BEARING DISTRICT. 



its two sides were coiitimuuis over tlie apex ot" tlie autioline before this was 

 removed by erosiou. Whether they were surface flows or intrusive sheets 

 is not known, since the rock composing- them is a chlorite-schist which has 

 lost all traces of its original structure. It seems probable that these sheets 

 are intrusive, while the amygdaloidal ones described below are effusive. 



The few sheet-rocks discovered, like the dike-rocks of the chstrict, were 

 originally diabases or basalts. They are now composed largely of amphi- 

 bole, chlorite, and altered plagioclase. They are either massive or schistose 

 in structure, with the massive phases porphyritic in texture. Some of 

 the latter are holocrystalline, while others originally possessed a glassy 

 gi'oiindmass. 



The rocks of the little hillock alread}- luentioued as existing in the 

 NE. i sec. 28, T. 47 N., R. 27 W., originally included both glassy and crys- 

 talline phases. In the former the glass which was formerly present in the 

 grouudmass has entirely disappeared. It has undergone change into a pale- 

 o-reen chloritic substance. The resulting rock is a dense, green, schistose 

 one, whose mass is speckled with bright, glistening surfaces of plagioclase. 

 In thin section small plagioclase crystals and groups of crystals, some fresh 

 and others changed to calcite, and small flakes of brown biotite, are seen 

 in a o-roundmass of smaller plagioclases, biotite wisps, tiny amphibole 

 needles, and granules of leucoxene embedded in a tibrous mass of chlorite 

 that is supposed to represent the original glass. 



The other sheet-rocks occm-ring in the same liilloek differ from the 

 one iust described mainly in the structure of their groundmasses. All of 

 them, whether amygdaloidal or not, possess an altei'ed diabasic groundmass 

 in no wise differing from the rocks that have been called uralitic diabases 

 and epidiorites. The aungdaloids occurring here ai-e dju-k-green porphy- 

 rites, with ])henocrysts of plagioclase and hornblende, and amygdules tilled 

 with magnetite and talcite in an altered diabasic groundmass speckled with 

 mao-netite. Under the micros(.'ope the feldspar phenocrysts are discovered 

 to be changed into calcite, and the green groundmass is found to be com- 

 posed of small plagioclases, pseudomorphs of calcite after plagioclase, green 

 chlorite, and leucoxene, with here and there some opidote and a few grains 



