50 J. D. Dana on the Homeomorphism 
In this type, the angles, as given in the table, are almost identical bigs 
those of Orpiment. The axes become for 
b=1 a=1 
Type Ia:d: ¢— 1-2876 : 1 : 11526 = 1: 0-77661 : 089526. 
Type Il, a:b: c= 18262: 1: 1:2030— 1 : 075405 : 090707. 
i | being 82 — 62° 12’ (e : e, f. 13, of Boacttih ad and $% (not observed) 
56’. They approach most nearly the unit domes of anhydrite. 
Tamale (FeO, TaO*) has very nearly the dimensions of Barytes 
(BaO, SO%), as seen in the table; and the fact is important, as it sus- 
tains the homeomorphism of tantalic and sulphuric acids. 
Brookite was first observed to be homceomorphous with Columbite 
by Hermann, It differs by four degrees in its domes from that species, 
and has its vertical axis about one-twelfth longer. 
In lumbite, it is of importance to note, that the face of twin com- 
eos: is a pee of the brachydome 2%, in which the basal angle is 
out 120°; and in ple td it is the brachydome 3%, in which the 
sei niigio' is about 120° 
Leadhillite has been von by the writer to have close relations im 
angle to Anglesite, in this Journal, vol. xvii, p. 210, Its dimensions, a8 
given in the table, ete still further this similarity of form. We fe 
serve remarks on the forms of Leadhillite for another occasion. 
Mascagnine sti widely from the other sulphates in its vertical 
prism, and therefore nines in its brachydome, while it agrees with them 
nearly in its macrodom 
da word on ne unit-octahedrons of the species of Ta- 
We ad 
ble IL The following are the angles for species in Sections |, 
III, and IV: 
Pyramidal ar Basal Angle. 
Section I—Barytes, - - 111° 38’ eaF 128° 36” 
nigldaite, - = 112° 13/ as 41? 128° 54’ 
Section It. Bare aces = = 1179 53/ 102° 58 107° 56! 
Brookite, - - 115° 49’ 101° 34! 111° 26’ 
Section ion - = 130° 49/7 120° 44’ 80° 22! 
Cotunnit 138° 227 123° 58’ 75° 48/ 
It will be observed dei there is an approximation to the angle 
of a regular octahedron only in one of the pyramidal angles of 
Section - I, and in the basal angle of Section III 
TABLE UL 
Angle of Vertical Prism, near 109° 28’. 










Marcasite, x - -»- | 1069 5} 64° 52’ 3 80° 20’ 15797 2118287) 
) ‘ a vote 6 AIS BRS cn BOD 14’} 80° 8’ | 1'7588:1: : 14798 : 
Avsstaliasitn © = ie 48” in 58° 52’ (2x) 78° 34! | 1e7t9s : 1: 14496, : 14496 
