66 REPOKT OP THE 



that tlie line of contact is merely zigzag, or truly suture-like, 

 while in other cases they become elongated prisms. The same 

 structure was long ago noted in the same formation, in Ohio, 

 by Dr. Locke* In New York it is found in the Niagara Lime- 

 stone, the Waterlime Group, and some of the higher rocks.f 

 These forms were termed by Prof. Eaton, Lignilites, from their 

 resemblance to woody fibre. In consequence of Mr. Vanuxem's 

 suggestion,! that this structure might be owing to sulphate of 

 magnesia, Dr. Beck subjected to analysis a specimen from, the 

 Niagara Limestone, and detected about 21 per cent, of carbon- 

 ate of magnesia.§ 



The limestones of this group are generally somewhat bitu- 

 minous, giving a brownish color and a fetid odor to the rock. 

 The bitumen at Christiancy's quarry in Monroe county, is so 

 abundant as to exude in the form of an oil, and float upon the 

 surface of the water. The bituminous exudation is very marked 

 in the Helderberg limestones of Northern Illinois. Black, bitu- 

 minous, shaly partings frequently occur between the strata in 

 the upper part of the group. 



Considerable hornstone appears in the formation at Raisin- 

 ville, in Monroe county, and also at Little Traverse Bay. Cu- 

 rious cherty concretions are very common. These sometimes 

 take the form of a perfect sphere, or ellipsoid of revolution, or 

 a gourd, and generally reveal at the centre, traces of some or- 

 ganic substance. These characters are supposed to appertain 

 to the " corniferous '' or upper portion of the group. At Brest, 

 Stony Point, Pt. aux Peaux, and some other localities, the bro- 

 ken strata abound in Strontianite, Dog Tooth Spar and fihomb 

 Spar. At Brest, Amethyst is found in limited quantity. Some 

 of the cherty nodules or pebbles at Mackinac, pass to the char- 

 acter of chalcedony and well marked agate. 



The economical importance of this group of rocks is very 

 great. They are everywhere useful for quicklime, and when 



♦Report of Geological Survey, Ohio, 1888, p. 230. 

 fHall, Geological Report, IV District, N. Y., p. 96. 

 iReport, in District, for 1838, p. 271. 

 $Beck, Mineralogy of N. Y., p. 69. 



