1 16 Prof. Chapman's Mineralogical Notes. 



planes of the rhombic prism in the crystal mea- ^S- I* 

 sured were more or less dull, so that D : D 

 could not be conveniently obtained by direct 

 measurement ; but, in other respects, the crystal 

 was very symmetrically formed, and its edges 

 were sharp and well-defined. The above value 

 closely approaches the angle D : D obtained by 

 Kengott. As deduced from Kengott's measure- 

 ments, this equals 128° 44' ; we may therefore 

 consider the angle to oscillate between 128° 30' and 129° 3(y. 

 A range of this kind, or one of even greater extent, might be 

 naturally anticipated from the variable constitution of the mineral. 

 I have never met with a crystal of staurolite in which the side 

 planes L were absent, and, in general, these are largely deve- 

 loped ; but in the curious compound crystal shown in fig. 2, the 

 planes in question are reduced j^~ g. 



to the breadth of mere lines. 

 They have been, unavoidably, 



enlarged in the figure. This 

 crystal comes from the neigh- 

 bourhood of Quimper. The 

 compound structure is exhibited 

 on only two of the planes of the 

 form I) ; so that, on one side, the crystal appears simple, as 

 shown in the drawing. 



2. Calcite. — The acute rhombohedron 4R in calcite, in the 

 simple or uncombined state, is comparatively a rare form. I 

 have noticed it, however, not unfrequently in the interior of 

 fossil shells or casts. To cite a few particular examples : in the 

 interior of a Spiri/er glaber from the carboniferous limestone of 

 Ireland ; in a Products punctatus from the same locality ; and 

 in a fractured ammonite of the Upper Jurassic period (A. Rupel- 

 lensis, D'Orbigny ; A. Bakeria, Quenstedt), from the vicinity of 

 La Rochelle. The angle over a polar edge =65° 50 7 (Contrast- 

 ante, Haiiy; e 3 , Dufrenoy; 3 if, Miller). I have also a spe- 

 cimen (fig. 3) of a calcareous nodule from the latter locality, the 

 surface of which is crystallized in the form 4R. Numerous and 

 very distinct lines of stratification or -pis. g # 



growth pass through the mass of the 

 nodule, as shown in the figure. This 

 specimen was found in a marly clay of 

 the same age as the English coral rag 

 (etage cor allien, D'Orb.) . In all of these 

 cases, the form has evidently resulted 

 from the gradual infiltration of calcareous 

 matter into a more or less resisting medium. 



