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CLAY SLATES AND FLAGS OF LAKE HURON. 



Alternating with the lower portions of the sandstones of Point aux 

 Barques, are strata of slaty sandstone, approacliing the character of slate ; 

 to which succeeds a compact, micaceous clay slate of a blue color. 

 This latter rock continues to occupy the coast for about thirty miles, or 

 from township twelve to township eighteen north, and rises in ledges 

 of fi'om five to fifteen feet. 



The slaty sandstones intervening between these clay slates, and the 

 overlying coarser sandstones are of a flaggy structure, in some of the 

 layers, and from these were obtained those fine flagging stones, which 

 have been extensively used, for three years past, for pavements in the 

 city of Detroit. Some of these strata are distinguished by ripple 

 marks. No fossils have been discovered in this formation. 



These slates and alternating sandstones may be considered as the up- 

 per salt rock of our State. They have been passed through in boring 

 for Salt at Grand Rapids, and found to yield strong supplies of brine. 

 At this point they are found also to alternate with beds of gyjjsum and 

 gyperous marls, as will appear by reference to the table of the strata 

 passed through, given on a subsequent page. The thickness ascer- 

 tained to these slates, at that point, is about one hnndred and seventy 

 feet. 



SOFT, COARSE-GRAINED SANDSTONE. 



» 



A series of sand-rocks, answering to this description, and generally of 

 a dark color, succeeds to the clay slates and shales last above described, 

 and has been penetrated at the borings at Grand Rapids, 230 feet. 

 There are, as yet, no data for ascertaining the entire thickness of this 

 series, since it does not make its appearance at any point on the coast of 

 the Peninsula, this rock evidently forming the bed of Lake Huron, near 

 its foot, and lying too deep for observation. In relative position, and 

 perhaps in character, this rock, or a porti(jn of the series, corresponds 

 with the lower salt rock of Ohio and Virginia, and is the rock from 

 which, in these States, the strongest supplies of brine are obtained. The 

 result of the borings in our own State, thus far, would seem to confirm 

 the opinion that this rock is the equivalent of the lower salt rocks of 

 those States. 



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