Genth.] 96 [Feb. 6, 



Limestone in the Pennsylvania Bank. 



Pale asli grey, very finely crystalline, rough to the touch like rotteu 

 stone, very friable and easily falling to powder. 



Its composition was found to be : 



Carbonate of Iron = 0.45 = 0.22 Metalliclron. 



" " Manganese = 0.06 



" "Magnesia 42.39 = 20.19 Magnesia. 



" " Lime 51.25 = 28.70 Lime. 



Quartz and Silicic Acid 5.03 



Alumina 0.82 



100.00 

 The atomic ratio between Magnesia and Lime^l : 1, shows it to be a 

 true dolomite. 



Another Variety of Limestone in the Pennsylvania Bank. 



Yellowish grey, soft, rotten, feels rough to the touch, sandy ; crystal- 

 line ; has a laminated structure. Its analysis gave : 



0.57 Metallic Iron. 



100.00 



The atomic ratio between Magnesia and Lime=l : 1.08 proves it also 

 to be a true dolomite. 



It is remarkable that the limestones and dolomites, of which I give the 

 analyses, contain almost the entii-e amount of silicic acid as quartz, only 

 a small quantity is present as soluble silicic acid and in combination with 

 alumina. If the limestones and dolomites are dissolved in acid, the 

 quartz remains often as a scoriaceous mass or in irregular sandy but 

 not rounded or water-worn grains ; sometimes it forms large coherent 

 slaty masses in the limestone, frequently filled with minute cavities, 

 previously occupied by rhombohedral crystals of dolomite. Similar 

 pieces found in the Pennsylvania Bank are white, like porcelain and 

 show the same cavities of rhombohedral crystals. Other varieties of 

 limestone in the Pennsylvania Bank have a still greater admixture of 

 quartz and are a real calciferous sand rock.* 



University op Pennsylvania, January 33d, 1874. 



* These analyses summed up about 100, most of them a little above, one or two a little 

 below, but all within the limits of unavoidable error; for better comparison I thought 

 it advisable to calculate them for 100.00, from the actual result obtained. (F. A. G-enth.^ 



