THE CHERTY LIMESTONE. 139 



gorge, the northern side of which is composed of a snow-wliite chert, while 

 the southern side is made up of a tremohtic minutely crystalline light gray 

 limestone. The limestone and chert appear here then to be separated into 

 two distinct layers, all of the chert overlying all of the limestone. The chert 

 at this place has a thickness something like 40 or .50 feet. Less than a 

 dozen steps south of the limestone are exposures of a Laurentian schistose 

 gneiss. 



The next exposures worthy of note are those lying to the east of Sun- 

 day lake, in the northwestern part of Sec. 15, T. 47 N., R. 4,") W., Michigan. 

 Here are extensive exposures of the siliceous slate member, immediately to 

 the south of which a number of test pits have uncovered chert and chert 

 breccia belonging to the Cherty limestone member. The principal point of 

 interest to be noted here is the abundant occuirence of i-ounded fragments 

 of yellowish white chert in the basal layers (8 to 10 feet in thickness) of 

 the slate series, the rock l)eing a chert conglomerate. The fine grained 

 cementing material, as well as the pebbles and bowlders, is largely chert; 

 so that the lower layer of the slate has derived most of its detritus from the 

 Cherty limestone member. In passing iipward the amount of cherty mate- 

 rial becomes rapidly less and the rock soon passes into the ordinary feld- 

 spathic quartz slate. In the northern part of Section 14 is a long east and 

 west exposure of the chert rock, immediately to the north of A\-hich is seen 

 recomposed material similar to that just described. Here, however, the 

 matrix of the chert congk>merate at the basement of the Quartz-slate mem- 

 ber is larg-elv of ordinarv detrital material. On the SE. V of Sec. 18 and 

 in the adjoining part of Sec. 17, T. 47 N., R. 44 W., Michigan, are the most 

 extensive exposures of limestone to be met with anywhere in the district. 

 There is here a bluff some 200 feet in height, running nearl}- half a mile 

 in a direction somewhat south of east. The extent of the rock here ex- 

 posed is so great as to indicate a total thickness of something more than 

 300 feet. This limestone is mostly quite fine grained and massive, and of 

 a o-rav color. Throusrhout tlie limestone there is contained nuich cherty 

 silica. Tliis silica is not oidy intimately mingled with the carbonate, as 

 shown by the thin section, but occurs in nearly pure layers interlaminated 

 with the limestone. The ijeculiar ridgy weathering all o\-er this exposure 



