244 THE MAKQUETTE IRON-BEAKIXO DISTRICT. 



PETROGRAPHICAL CHARACTER. 



Macroscopicai. — Petrograpliically the diftereut exposures are very similar. 

 The formation is not a pure dolomite, Ijut is a cherty dolomite interstratified 

 with layers of slate, graywacke, and quartzite, with all gradations between 

 the various mechanical sediments and between these and the pure dolomite. 

 In some exposures the pure dolomite does not constitute more than a third 

 to a half of the belt. The interstratified slates and quartzites are of the 

 same character as those of the Wewe slates and Ajibik quartzites, except 

 that they are apt to be more or less calcareous. The dolomite beds vary 

 in thickness from only a few inches to many feet. But even the solid belts 

 of limestone usually contain very thin layers, which in places are in part 

 fragmental, but which are usually wholly or in large part secondary chert. 

 In color the dolomite a' aries from nearly pure white to dark-brown, depend- 

 ing upon its purity, and between these colors are various shades of buff, 

 purple, pink, and red. As the interbedded slates and quartzites also have 

 a wide variation in coloring, the ledges of the formation are very different 

 in their aspects. 



The dolomite varies from aphanitic to coarsely crystalline. Upon the 

 Tveathered surface the pink and red varieties usually have a dark-brown 

 color, due to limonite. This indicates that the carbonate carries a consider- 

 able quantity of iron, the oxidation of which has produced this outer dark- 

 colored skin. Where the dolomite is most coarsely crystalline, as, for 

 instance, at jMorgau Furnace, it sometimes contains belts from a fraction of 

 an inch to 4 inches wide, largely composed of pink, coarsely crystalline, 

 and evidently rearranged dolomite (PI. VII, lig. 1). As a consequence ot 

 weathering, the bands of original sedimentary quartz and of secondary 

 chert protrude, giving a peculiar rough, ridgy appearance. 



Microscopical. — The rocks of the Kona dolomite comprise coarseh^ and 

 finely crystalline dolomite, cherty dolomite, quartzose dolomite, argillaceous 

 dolomite, dolomitic quartzites, dolonaitic slates, dolomitic cherty quartzites, 

 and dolomitic chert. 



The compact and apparently least altered, purer rock has a background 

 consisting of fiuel)' gramilar dolomite, separate granules of which are 



