» 
a 
a 
86 J. D. Dana’s Mineralogical Contributions. 
sum is 111° 42’, in Heulandite, 98° 40’. When the axes are equal 
in two cases, the inclining of the vertical axis in one, diminishes 
the distance of its apex from the plane of the lateral axes, and con- 
sequently, the clinodiagonal prisms, and the octahedral forms 
on the obtuse basal edges and angles of the fundamental prism, 
become more and more flattened. This is the main source of the 
differences in angle between Gypsum and Heulandite. It shows 
us that in the oblique systems, we have to, look to the relations 
of the axes, rather than to all the angles, in tracing out the re- 
semblance of form among species. 
Another inference from this case of isomorphism is that al- 
though some cases show that cleavage is a subordinate character, 
we may err in comparing crystals of different species, if we do 
not give due importance to the cleavage directions. 
8. Homeomorphism of Brookite and Columbite. 
Through the kindness of Mr. George J. Brush, I have recent- 
ly received a specimen of Brookite from the Ellenville lead mine, 
Ulster Co., N. Y., occurring implante 4. 
ona group of quartz crystals. The 
mine is situated in the Shawangunk ( 
grit, a Silurian rock. The crystal has 
a striking resemblance in habit to that 
of the Columbite of Middletown, Ct., 
even to minor points in the arrange- 
ment of the planes. The form sug- 
gested at once the homeomorphism of 
the species, and this is closely sustained 
by the angles 
Bie 
= 
es, 
Ni 
Brookite. Columbite. 
Angle of prism J «P) 99° 50’-100° 30’ 100° 40’ 
0: 2%(0P : 2P'=) 117° 54’ 119° 40’ 
G20 -¢ 094438 : 1: 084158 08675 : 1: 08291. 
The isomorphism is as close as between Celestine and Barytes. 
It is of interest to compare the Columbite series (which in- 
cludes Euxenite, Samarskite, Wolfram) with the Barytes series. 
The axes are as follows: ; 
: Columbite. Heavy Spar. 
a:b7¢ 0°8778 : 1 : 0:8292 13127 : 1 : 081413, 
Now it is apparent that @ in Heavy Spar is 14 times a of Col- 
umbite. ‘The relation is consequently a simple one—and so sim- 
nations on the base of the prisin. : 
_. Such remarkable cases of homeomorphism almost show that 
similarity of form may exist irrespective of the elements con- 
__ cerned, or of any numerical ratio in the atoms of the elements 
