16 E. §. Dana on the Datolite from Bergen Hill, N. J. 
specific heats of compound gases, we should at the same time 
expect, would be greater than that of a simple gas. 
Art. IV.—On the Datolite from Bergen Hill, New Jersey; by 
Epwarp 8. Dana. With Plate L 
THE Bergen Hill tunnel is famous for the abundance, beauty, 
and variety of the minerals which it brought to light. Datolite, 
pectolite, calcite, analcite, apophyllite, natrolite, stilbite, and 
others were obtained there during its excavation in a degree o 
perfection rarely equaled by the pinion of any other local- 
ity. The crystallizations of datolite are especially remarkable; 
some of the surfaces covered with the brilliant crystals being 
eighteen to twenty-four inches in length.* 
e crystals are in general not over a third of an inch across, 
though they sometimes have a diameter of one inch. Those of 
a single specimen have always entire uniformity of habit. The 
datolite is associated on different specimens with most of the 
other species found at the same locality, but it was not found 
possible to obtain any facts which would throw light upon the 
influence of the associated minerals on the crystalline form. 
Among the varied forms, four different types may be distin- 
ished. 
a Figure 1 represents a very common and characteristic form. 
The crystals here are very thin, wedge-like, and are attached to 
the mass of rock approximately by the extremity of the clino- 
diagonal, though varying from that of the diagonal terminating 
between 2 and 2 on one side, to that between —4 and —4 on 
the other. From the position and shape of these crystals, they 
offer an unusual number of surfaces for the reflection of the 
light, and hence give the specimen a brilliant sparkling aspect, 
* — specimens of this kind occur in the collection of Mr. Haines of Eliza- 
