28 LOWER PENINSULA. 



and soft, their calcareous cement seeming to have leached out while 

 the deeper sited strata are hard and rich in calcareous matter. 



The rock contains some casts of bivalve shells and of gastero- 

 pods ; its fissures and druse cavities are filled with strontianite, 

 coelestine and calcspar. The sand rock reposes on a hard, com- 

 pact dolomite rock, mottled with light and dark blue cloudy specks 

 resembling castile soap. The dolomite is composed of 



Per Cent. 



Carbonate of lime 54 



'' " magnesia 42 



Quartz sand 4 



The lower hard cemented sand-rock beds contain 



Per Cent. 



Carbonate of lime 46 



White quartz sand 54 



Sand-rock ledges of a somewhat different character from those 

 mentioned, but evidently equivalent with them, are exposed on 

 both sides of a road passing the north end of Ottawa Lake. The 

 rock is hard, fine-grained, of dark bluish, or, in weathered condition, 

 of ferruginous brown color, and contains 65 percent of cal- 

 careous matter by 34 per cent of quartz sand. It is quarried 

 for building purposes ; intermediate between the harder layers are 

 seams of a coarse-grained, softer sand rock with only a small pro- 

 portion of calcareous cement. Inclosed in these softer seams, I 

 found dermatic plates of macropetalichthys. South from there, 

 across the State line, near Sylvania, in Ohio, a similar seam of 

 sand rock is found intercalated between the upper and lower Hel- 

 derberg limestones ; and from the geological reports of [Ohio we 

 learn that throughout the entire Helderberg area of that State a 

 sand-rock deposit is constantly found in such position. In some 

 localities the sand rock seems to be replaced by an oolith. In the 

 quarries of Plum Creek, near Monroe, and near Little Lake, in Bed- 

 ford township, the mottled dolomite rock which lies at the base 

 of the sand rock in the Ida quarries. Section 16, is found in 

 the same characteristic form, but instead of sandstone, in the two 

 mentioned localities, oolithic rock beds are superimposed. Their 

 chemical composition is: carbonate of lime,"6i percent, and carbo- 

 nate of magnesia, 37 per cent. The thickness of this sand-rock stra- 



