HURONIAN UNCONFORMITY IN MICHIGAN 579 



similarity of the arkose-conglomerate of Little Lake to that at the 

 base of the Gwinn series extends to the pebble content. Rounded 

 fragments of dolomitic siliceous slate, and gray quartzite are 

 common to both localities, but the bowlders of granite and green 

 schist which occur in the conglomerate of the Gwinn district were 

 not observed in the exposures at Little Lake. 



UPPER (pRINCETON) SERIES 



The upper series, so far as represented at Little Lake, com- 

 prises a higher horizon of red- and gray-banded quartz slate and 

 slaty quartzite grading down through banded quartzite and 

 massive non-bedded quartzite into a basal conglomerate. 



Conglomerate. — The contact of the upper and the lower series 

 is exposed at locahties B and C (see figure). At locality B this 

 contact is distinguishable only on careful examination. The 

 base of the upper series on weathered exposures is not conspicuously 

 dissimilar to the underlying arkose except on freshly fractured 

 surfaces which reveal, in contradistinction to the underlying 

 sericitic, quartz-feldspar rock, a dense, hard matrix of quartzite 

 holding pebbles of vein quartz of sizes less than an inch in diameter. 

 At locality C, however, all doubt of the unconformable relations 

 of the arkose-conglomerate and the overlying series is dispelled. 

 The change from arkose to dense, black, vitreous quartzite is 

 abrupt at a wavy contact of knife-like sharpness. In addition to 

 the quartz pebbles observed at locality C there are pebbles of chert 

 and large bowlders of the underlying arkose above one foot in 

 diameter. The arkose bowlders are much softer than the embed- 

 ding matrix of quartzite and weather out to form characteristic 

 pit-like depressions. The full thickness of the basal conglomerate 

 is not exposed at locality C, but at locality B it is apparently only 

 six feet. At C only about four feet are observable. 



Quartzite and quartz slate. — There are three distinct main phases 

 of this series, viz., (i) a massive phase associated with the basal 

 conglomerate, grading upward into (2) a banded phase which in 

 turn is over-lain rather sharply by (3) beds of gray- and red-banded 

 quartz slate. Although these three phases correspond to definite 

 stratigraphic horizons, considerable difficulty is experienced in 



