Luther — Geology of the Salt District. 



211 



The limestones are generally thin-bedded and laminated or shaly, though 

 some layers are a foot or two thick and compact but quite brittle, breaking 

 with a metallic ring, and splitting into concohoidal slabs. 



AVhen first quarried in the eastern part of the district, they are very dark- 

 colored and bituminous, thin partings of carbonaceous matter separating the 

 layers. After exposure they become a light ashen grey. 



In the western counties, much of the limestone, when freshly exca- 

 vated, has a dark reddish brown color that changes to a light pink grey on 

 exposure. 



Evidence that these rocks were deposited in water that contained salt 

 nearly to the point of full saturation, is found in the hopper-shaped forms 

 that occur at several horizons, and in the numerous cavities of the porous 

 limestones which it is now known were once filled with salt. 



The larger cavities are irregular in size and shape, and are more common 

 on the surfaces of the layers, but others of a cellular character and from a 

 microscopic size to three-eighths of an inch in diameter penetrate the rock in 

 all directions communicating with each other and giving the limestones the 

 pecular appearance indicated by the name applied to them by Vanuxem — 

 " vermicular." 



Fragments from a layer of very dark and apparently compact limestone 

 in the Livonia shaft thirteen feet above the rock salt bed, changed on 

 exposure to brownish grey, and after being immersed in the water for a few 

 hours was found to contain numerous tubular cavities about one sixteenth of 

 an inch in diameter. 



The limestones also show many cracks, not like pressure cracks or exten- 

 sive joints, but shallow and like those seen in sun-dried mud. When freshly 

 excavated they are filled with salt, gypsum or black carbonaceous matter. 



No fossils have been found in the red shales below the salt beds nor in 

 the shales and limestones above described. 



The Waterlime Formation. Next above the gypseous deposits are 100 to 

 150 feet of strata composed almost entirely of limestones, that constitute the 

 " Magnesian deposit " of the early geologists. They are of the same general 

 character as those in the beds below. The principal and perhaps the only 

 reasons for separating them are the absence of the gypsum beds and the occur, 

 rence of the earliest traces of the existence of life subsequent to the Niagara 

 epoch. The appearance of the little crustacean Leperditia alta in this horizon 

 is evidence that the precipitation of the enormous quantities of salt and 



