62 Mr Haidinger on the Regular Composition 



inclined to the axis, disappears, and then the compound 

 group assumes the appearance of a combination of prisms and 

 pyramids belonging to the prismatic system. 



The most interesting, however, among the more complex 

 cases of regular composition, relative to the present species, is, 

 when it takes place at the same time, and to the same extent 

 of the additional individuals, parallel to all the faces of the 

 fundamental pyramid. The result of this composition is re- 

 presented in Fig. 8., a remarkably elegant form, the symmetry 

 of which would be alone sufficient to prove, that it belongs 

 to the pyramidal system, even though we should remain ig- 

 norant of the absolute measures of its angles. It has been 

 found in the Cornish and the Bohemian tin-mines. Its crys- 

 tallographic designation, according to the method of Mohs, is 

 P + l. [P + co], {P}. 



Another species, well known for the frequency of its occur- 

 rence in regular compositions, is the peritomous Titanium-ore. 

 The composition takes place parallel to a face of the pyramid 

 P — 1, the angles of which are =128° 41 7 and 76° 31', according 

 to Haiiy ; the inclination, therefore, of the axes of the two indi- 

 viduals, (Fig. 9.) is =104° 29'. Although this kind of compo- 

 sition be exactly the same as the one in pyramidal Tin- ore, yet 

 the apices of the combinations are less frequently observed, and 

 the geniculated appearance remains to ascertain the existence of 

 regular composition. Upon the situation of the plane in which 

 the individuals meet, and the observation that this plane is fre- 

 quently parallel to a face of crystallization, Haiiy has grounded 

 his first determination of the angles relative to the fundamental 

 pyramid of this species, forms which he had never seen before; 

 and though afterwards corrected by the more accurate mea- 

 surements of Mr Phillips in regard to the angles themselves, 

 yet the method which he followed deserves a praise propor- 

 tionate to the degree of importance attached to it by Haiiy 

 himself. Particularly among the varieties from Buytrago and 

 from St Yrieix, we meet with a continuation of the law, accord- 

 ing to which two individuals are joined to a third, forming 

 thick crystals like the groups represented in Fig. 10. It has 

 been observed by Haiiy, and this is by no means rare among 

 regularly composed minerals, that in these compositions the 



