THE AMERICAN MUSEUM GUIDE LEAFLET 
galena as a decomposition product of the latter and is often mined with 
it and other ores. 
A very striking and beautiful series in Case 19 is that which rep- 
resents the mineral crocoite, the lead chromate. The bright hyacinth 
red and orange color, which constitutes one of the chief characteristics 
of this mineral, is affected by long exposure to the light and consequently 
the suite of specimens is covered with a black cloth which should be 
removed in order to view this exhibit. One of the most common and 
important of the sulphates is gypsum (Case 19), the hydrous sulphate of 
calcium. The exhibited suite of specimens is remarkable for the size 
and quality of its crystallized examples both as single individuals and in 
large groups. Among the rarer species which are included among the 
hydrous sulphates are many specimens which combine great beauty of 
color with interesting structure, features which tend to make this one 
of the most attractive sections of the collection. 
TUNGSTATES, MOLYBDATES 
Although represented by very few minerals, this division of the 
classification contains three important species, wolframite the tungstate 
of iron and manganese, scheelite the tungstate of calclum, and wulfenite 
the molybdate of lead. These are all important minerals from a com- 
mercial point of view, because they furnish us with the rare metals 
tungsten and molybdenum which are used to make special steels of a 
high grade of strength and durability. The series which includes these 
three, as well as many rarer tungstates and molybdates, will be found in 
Case 20. 
THE MINERALS OF MANHATTAN ISLAND 
Manhattan Island offers the somewhat unique case of a limited 
area of mineral-producing rocks where excavations have been carried 
forward to such an extent that practically all of the crystalline rocks 
which underlie the drift deposits have been exposed at some time, and 
most of this area has been laid bare within a fairly recent stage in the 
development of the City. Asa consequence of this unusual activity in 
excavation, much is known concerning the minerals which occur in the 
local rock formations, and the local collections made from these rocks 
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