ALGO:N"KIAiSr ROOKS OF STUEGEON RIVER TONGUE. 481 



PETROGRAPHICAL DESCRIPTION. 



Ill thill section the marbles appear as very close-grained aggregates 

 of ealcite and dolomite, usually untwinned, but occasionally twinned in 

 the ordinary manner of these minerals. Here and there among the car- 

 bonates are rounded quartz grains, but the greater portion of this min- 

 eral appears to have crystallized in situ between the ealcite and dolomite 

 individuals. 



All the marbles are of the same general character. They differ only 

 in the quantity of silica jDresent and in the presence or absence of the tiny 

 dust grains producing the color. The schistose varieties owe their schistos- 

 ity to the elongation of their components. 



The quartzites and slates iiiterbedded with the marbles possess no 

 unusual characters. They are similar to the corresponding rocks inter- 

 stratified with the Marquette dolomites. The conglomerates interstratified 

 with the dolomites, slates, and quartzites are of two kinds. One is com- 

 posed of marble and slate fragments cemented by quartzite, and the other 

 of small granite pebbles embedded in granite sand. The latter are evi- 

 dently composed of the detritus of the granites underlying the dolomite 

 series, while the marble-bearing conglomerates, or perhaps more properly 

 breccias, are interformational beds conformable with the beds below them, 

 and also with those above. They are similar in every respect to the inter- 

 bedded breccias in the Kona dolomites on the Marquette range. 



SLATES AND SANDSTONES ON THE STURGEON RIVER. 



The rocks in the SW. 5 sec. 34, where the road to Sagola crosses the 

 Sturgeon River, are placed in the dolomite formation, although they differ 

 somewhat from that portion of the series described. These rocks are white 

 calcareous sandstones, that look very much like the Potsdam sandstone 

 where it overlies limestones, and a light-green slate, which near joint 

 planes and other cracks has a light purple color. According to Dr. J. M. 

 Clements, who visited the spot, the slate overlies the sandstone. "The 

 river," he writes in his notebook, "gives a section through these rocks, and 

 makes the strike seem to be N. 35° W., dip 50° N. It appears to me, how- 

 ever, that the true strike is about N. 85° E., and dip 40° S." If these rocks 

 belong to the marble series, they constitute its upper part. The slate 

 closely resembles some of the slates in the Kona dolomite formation of the 



MON XXXVI 31 



