of Mineral Species of the Trimetric System. A5 
taken as the unit vertical prism. A prism with the longer lateral 
axis half as long, has the angle 93° 8’, and this approaches the 
prism of Andalusite; and as the frequency of occurrence of a 
plane is no sure proof that the plane is necessarily of the funda- 
mental series, we may with some reason assume the prism of 
93° 8’ to be the fundamental one. But Staurotide forms twins in 
two directions, or parallel to two planes, and neither of these 
planes, referred to the above fundamental forms, has a simple ra- 
tio or expression, and this, notwithstanding the general fact that 
the faces of composition are of the highest value in ascertaining 
the directions of axial sections: moreover, one of the planes has 
the unusual symbol 22 if referred to the prism of 129° 20’, and 
; # if referred to that of 93° 8’. Now, if instead of halving the 
longer lateral axis, we take two-thirds for the new axis ¢, then 
the expression is of the simplest kind in every respect.’ The fol- 
lowing are the angles and symbols of the planes according to 
these three methods :— 
A—Prism J=129° 20’; W= 69° 16’; 3% (one face of com- Sle : ae 
position) = 83° 24’; 23 other face of composition. 
B.—Prism J= 93° 8’; 27 69° 16/; 3% (one face of compo- 
sition) = 88° 247; & 3 other face of composition; *1i==108° 0°7239 : 1: 105617 
12’; 1y¥== 111° 107, 
C.—Prism 7— O44’. 17 —89° Tel: 1 3 ei 
ponte) = 66° 4’ i sahek cant alien Mame wes t Pept 
Tn the last, the planes, and the faces of composition have all a 
unit ratio, and it affords the simplest possible view of the crystal- 
lization. Whether regarded as the fundamental form or not, the 
relation to andalusite is shown by the fact of the two belonging 
to one and the same series or system of ratios. 
Topaz has I: [=124° 19’ and 55° Al’, and 73 : 23=86° 52’ and 
93° 8’. The two prisms might either be taken as the fundamen- 
tal, with nearly equal propriety. If the first be so taken, and the 
macrodome of 58° 31’ be the unit one,.the axes are a: b:c= 
189774 : 1-05625 : 2 (=1-7587 : 1: 18936), a being treble what 
it is in Table I, and b double, the 5 also becoming ¢ or the longer 
lateral axis, If the unit macrodome is that of 96° 2’, the axes are 
the same except that @ is half as long. 
Lievrite is usually considered as having for its fundamental 
Vertical prism, a prism of 111° 12. Now this angle is near 109° 
14’ for Staurotide, (type C); and taking 2% as the vertical prism 
I, the angle is near that of Andalusite. Moreover the species has 
hear relations in its domes to the species of Table I, and none to 
those of Table IL. Besides, in composition it resembles Anda- 
lusite and the allied species, in having less oxygen in its silica 
than in its bases. These facts afford some reason for placing the 
Species where it stands in Table I. 
