134 MR HAIDINGER on the Natural-Historical 



But this variety, likewise, contains real faces of cleavage situated 

 in other directions, which intersect the former in oblique angles. 

 They are but slightly marked, and rather incohering, wherever 

 the brightest green colour is joined to a more perfect appearance 

 of the laminae ; whereas they assume a higher degree of distinct- 

 ness in the darker blackish-green varieties, into which the former 

 pass without in the least changing their parallel position, exact- 

 ly as is mentioned by HAUY. There are two such faces of clea- 

 vage, intersecting the faces of composition at an angle of 152, 

 while they intersect each other at an angle of 1 24. Accurate mea- 

 surements, taken by means of Dr WOLLASTON'S goniometer, have 

 yielded the angle formed by the two faces of cleavage in hemi- 

 prismatic Augite-spar 124 13', very little different from 

 124 1-5', the result of NORDENSKIOLD'S observations, both of 

 these being different from 124 34', the angle indicated by HAUY. 

 If, according to the crystallographic method of Professor MOHS, 

 we designate this prism by (Pr + 00 )\ which sign it receives by 

 being considered in connection with the rest of the simple forms 

 in the same species, the face of composition, parallel to the 

 greater diagonal of this prism, or to the lesser one of P -f- oo , 

 will be expressed by the sign P r + 



The regular composition of the twin-crystals of the present 

 species, is parallel to the very same face Pr-f- o s of which, 

 among many well known examples, I shall only mention the va- 

 rieties found in the basalt of Bohemia. Several varieties of com- 

 mon Hornblende present a similar composition, as, for instance, 

 that contained in the zircon-syenite from Friedrichsvarn in Nor- 

 way ; but none of the varieties, not even those of green Diallage, 

 exhibit these faces of composition so perfectly smooth and even, 

 as the greenish-black Hornblende from the Kiennerud mine near 

 Kongsberg in Norway ; and yet this has never been considered 

 as a variety of Diallage. 



