of the Trimetric System. 51 



A.— Prism /= 129° 20'; li = 69° 16'; U (one face of ) a. : b : c 



V 1.4478 : 1 : 2.11233 



composition) = 88° 24'; % % other face of composition. 



B. — Prism /= 93° 8'; 11 = 69° 16'; \% (one face of com "] 



position) = 88° 24'; % | other face of composition; II \ 0.7239 : 1 : 1.05617 

 = 108° 12'; K = lli° 10'. J 



C— Prism /= 109° 14'; li = 69° 16'; 1« (one face of ) , , Mrt 



. . \ ' , . . . , M.4478 : 1 : 1.40822 



composition) = 88 24 ; 1, other composition face. \ 



In the last, the planes, and the faces of composition have all 

 a unit ratio, and it affords the simplest possible view of the 

 crystallization. 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 /: 1= 124° 19' and 55° 41', and i2 : i r 2 = 86° 52' 

 and 93° 8'. The two prisms might either be taken as the fun- 

 damental, 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 = 1.89774 : 1.05625 : 2 ( = 1.7587 : 1 : 1.8936), a being 

 treble what it is in Table I, and b double, the b also becoming 

 c or the longer lateral axis. If the unit macrodome is that of 

 96° 2', the axes are the same, except that a 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 i% as the vertical 

 prism i, the angle is near that of Andalusite. Moreover the 

 species has near relations in its domes to the species of Table I., 

 and none to those of Table III. Besides, in composition it 

 resembles Andalusite 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. 



The following are notices of other species in Table I. : 



Chondrodite has for the summit angle of ll in its three types 

 68° 32', 64° 54', 70° 29', giving as the mean 67° 58', from which 

 the mean for \i (taken as It in the table) is 106° 52', and the 

 extremes 103° 28' and 109° 26'. The angle for II in the New 



