GUIDE TO THE MINEKALOGIC COLLECTIONS 129 



less frequent, as are also granular massive forms. The com- 

 moner colors include a wax yellow, orange to bright red and 

 brown; an olive-green variety is rather 

 rare. 



Wulfenite occurs in veins associated 

 with other ores of lead particularly 

 vanadinite and pyromorphite. In the 

 United States it is principally found in Arizona and New Mexico 

 though smaller deposits have been found in Massachusetts, 

 near Ossining N. Y., in Pennsylvania, Missouri. Wisconsin, 

 Nevada, Utah and California. 



HYDROCARBON COMPOUNDS 



With few exceptions, hydrocarbon compounds are not homo- 

 geneous substances and hence are not to be classed as definite 

 mineral species. On the other hand, several substances belong- 

 ing to this division have acquired so much importance from an 

 economic point of view that it is thought best to briefly describe 

 them here. Amber 



Amber occurs in irregular masses which break with a concoidal 

 fracture. It has a resinous luster, is usually yellowish in color 

 and is transparent to translucent. Amber is of vegetable origin 

 and is derived from the fossilization of gums or resins, a fact 

 which is frequently shown by the presence of insects in it. 



It is found in Denmark and Sw r eden, on the Prussian coast 

 of the Baltic and in Kussian Baltic provinces. It is used for 

 jewelry and for mouthpieces of pipes. 



Petroleum 



Members of this series grade from a thin yellow fluid to dark 

 brown or nearly black viscid oils; the greenish brown colors are 

 the most common. The density also varies and it may be gen- 

 erally stated that the light varieties are richest in volatile con- 

 stituents while the heavier and darker kinds produce the ben- 

 zins on distillation. 



Petroleum is found in rocks of various ages from the Lower 

 Silurian to the present epoch but is most abundant in argilla- 

 ceous shales, sands and sandstones. Considerable petroleum is 

 furnished by the regions of western Pennsylvania, southwestern 

 New York and Ohio. It is also largely produced in the neigh- 

 borhood of the Caspian sea and occurs in many other localities. 



