236 Field Columbian Museum — Geology, Vol. i. 



Measured. Calculated. 



0445= 40° 39° 4' 



4045 = 65 64° S3 J A ' 



4153 = ! 7° 14° 28^ ' 



45I~3 = 78° 78° 5' 



5^43 = 19° 18 7' 



3145 = • 33° 33° 50' 



4l35 = So" 49° 2 3 ' 



5~i43 = l6 ° l6 ° 



The planes of r are further identified by their evident parallelism 

 to the cleavage planes. 



An interesting peculiarity of the specimen is the surface differ- 

 entiation of the planes. Thus the planes of the scalenohedron v and 

 the negative rhombohedron / are flat and bright ; those of the scalen- 

 ohedrons n and w are striated parallel to the adjoining edges of the 

 unit rhombohedron, and the planes of the unit rhombohedron and 

 the rhombohedron M are rough. The differentiation closely follows 

 and beautifully illustrates the law that crystallographically equivalent 

 planes are similarly affected. An examination of the rough planes 

 reveals but little in the way of symmetrical etching figures. The 

 roughness is caused chiefly by delicate pitting, which is also wholly 

 irregular. There are to be seen occasional pits, however, which are 

 larger than the others and which have a definite pyramidal shape. 

 They are different in form from the etching figures usually produced 

 artificially on calcite by hydrochloric acid. The boundaries of the 

 pits lie parallel to the bounding planes of the unit rhombohedron. 

 Their character is illustrated in the accompanying figure (Fig. 10). 



Fig. 10— Growth figures on plane of +R. Calcite, Joplin, Mo. 



