48 MINERALOGY AND LITIIOLOGY. 



exists in most of the bog ores of iron, as is proved by treating them with 

 hydrochloric acid. In some of our bogs this ore can be seen in process 

 of formation. A sluggish stream, flowing perhaps from a chalybeate 

 spring, runs into a marshy spot, and the top of the water is seen covered 

 with an iridescent slime, — a result of the oxidation of the iron in solution 

 to the insoluble sesquioxide, — while the bottom is covered with a yellow 

 deposit of sesquioxide of iron. 



Limonite is a common constituent of rocks. Sometimes it exists as 

 an original constituent, as where it forms the cementing material of con- 

 glomerates; or, again, it results from the hydration of other oxides of 

 iron, under which circumstances it appears as a yellow, semi-transparent 

 substance surrounding an opaque, unaltered core, if the decomposition 

 is incomplete. 



I would refer to a number of analyses of hydrous iron ores in Jack- 

 son's report; but, as they have no mineralogical significance, they are 

 not reproduced here. 



35. PSILOMELANE, WaD. 



Impure, hydrous manganese oxides. Wad is found in many of the 

 deposits of bog iron ore. Sometimes it is sufficiently compact and pure 

 to be called psilomelane, but most of it is very impure, being much con- 

 taminated with iron oxides, organic matter, and other impurities. It is 

 recognized as a manganese oxide by its black color, and its manganese 

 reactions before the blow-pipe. Psilomelane is found at Winchester, 

 with the other manganese minerals. 



36. MOLYBDITE [Mo O3] . 



This mineral is a result of the oxidation of the sulphuret of molyb- 

 denum or molybdenite, and occurs in connection with it. It is found in 

 the cavities of the veins of molybdenite in Westmoreland in considera- 

 ble amount, and less abundantly with the other deposits in Landaff and 

 Franconia. As it occurs in our state, it is an earthy, yellow mineral, 

 filling cavities or incrusting the sulphuret. 



This mineral is easily recognized by heating it with the blow-pipe upon 

 a piece of charcoal, when it is volatilized, coating the coal with a white 

 sublimate, which, when touched for an instant with the reducing flame, 

 is changed to a beautiful blue color. 



