CARBONIFEROUS LIMESTONE. 105 



in the bed of Rifle River. Some vegetable remains can also be ob- 

 served. The thickness of this limestone may reach 8 or 10 feet ; 

 above it is a seam of coarse-grained, drab-colored, rusty dolomite 

 rock, which is crowded with casts of Spirifer Marionensis and va- 

 rious kinds of Lamellibranches, Terebratula Eudora ? Retzia, and 

 others. Purer calcareous beds, nearly all composed of shell frag- 

 ments like those at Oak Point, are interstratified here, and higher 

 we find blue argillaceo-arenaceous limestones, of an absorbent, 

 porous character, which contain somewhat abundantly the Pro- 

 ductus species mentioned several times before, but few of other 

 fossils. A hard, calcareous bed with flint concretions overlies 

 them, and then follow arenaceous shales and harder sand-rock 

 ledges of a bright red, or yellowish green and red variegated 

 color ; some of the layers are in brecciated condition. The total 

 thickness of the red layers may be 15 or 20 feet, of the entire sec- 

 tion exposed in the river bed, 50 feet. Further down stream, the 

 strata disappear under the drift, and no rock is exposed in the bed 

 of Cass River until Tuscola village is reached, where small out- 

 crops of the coal measures are observed. Up stream, between 

 Brown's farm and Indian Rapids, by undulations of the rock beds 

 the Waverly sandstone becomes sometimes bent downward into a 

 synclinal trough, and the intermediate depression is filled out with 

 the higher fossiliferous limestones, containing Zincblende concre- 

 tions. 



Next above the described rock series the horizon of the gyp- 

 sum deposits begins, but a direct superposition of the strata is 

 nowhere observed, and the gypsum, as I have intimated before, is 

 only found in local deposits, many places on the same geological level 

 with these bearing no signs of it whatever. The largest exposures 

 of the gypsum formation are found on the shore of Lake Huron, 

 at Alabaster Point, where beds of pure gypsum, covered by only a 

 few feet of drift, lie quite near the surface. The gypsum beds are 

 deeply eroded by the solvent action of atmospheric waters ; during 

 the glacier time, also, much of the soft rock became destroyed, or 

 intersected by large, deep grooves which are now filled up with drift 

 material. The practical quarryman in search of gypsum is 

 guided in his explorations by certain surface indications. 



The surface in those places where the gypsum is close under it 

 is full of small pot-holes and intermediate hillocks, and he knows 



