GEOLOGICAL STRUCTURE. I3 



bluffs ; above them follow wcl 1-st ratified sand beds on which the 

 red clay beds rest in almost shale-like, laminated form, amounting 

 to a thickness of from 30 to 50 feet ; usually this clay belt is 

 divided by interstratified sand layers into several bands. Certain 

 seams of the clay are soap-like, absolutely free from coarse par- 

 ticles ; others contain numerous pebbles, or are mixed with various 

 proportions of sand. The impermeability of this thick clay mass 

 to water causes the issue of copious springs along the bluffs above 

 the horizon of the clay. The upper part of the bluffs, which locally 

 attain a height of 200 feet, is formed of alternating layers of sand 

 and gravel in irregular, discordant stratification. The gravel beds 

 between the sand strata are often five or six feet thick in the 

 centre, and wedge out on both sides into linear seams, which, per- 

 haps, further on, may be seen expanding again, or entirely vanish. 

 Large boulders are mixed into the strata at all heights, but in cer- 

 tain levels of the upper part of the bluffs, boulders are more abun- 

 dant, and stratification becomes obscure. On top of the bluffs 

 heavy masses of fine sand are deposited, which partly have the 

 character of dunes, partly are stratified, aqueous deposits, but of 

 much more recent origin than the lower portion of the bluffs. 

 Exactly the same exposures of the drift as on the shore of the 

 mainland of the peninsula, are seen on the west side of the Mani- 

 tou Islands, in connected sections of about 150 feet in height. 

 Sleeping Bear Point, a promontory opposite the Manitou Islands, 

 on the main shore, said to be of 500 feet elevation, is remarkable 

 for its barren, naked surface of a fine, loose sand, without any 

 vegetation. Its base is composed of the same drift layers as the 

 bluffs further south, of which it is a direct continuation. In many 

 other localities of the interior, the same well-stratified red-clay 

 beds are uncovered, always holding a position below the principal 

 coarse boulder drift. The banks of Cass River, near Vassar, in Tus- 

 cola County, or the bed of Muskegon River near Big Rapids, give 

 occasion to see the clay beds in their relative position to the 

 other drift strata. In a cut of the railroad between the Big Rapids 

 and Muskegon, close to the first-named village, an interest- 

 ing section through the drift is laid open. Deepest, at the 

 bottom of the cut, boulder drift mixed with stiff bluish clay is 

 exposed. The large boulders, as well as the smaller pebbles, are 

 plainly marked with drift scratches. Above them are about 30 



