236 FIELD COLUMBIAN MusrEuM—GEOoLoGcy, VOL, I. 
Measured. Calculated. 
° 
ee fe — oe (oh 0 Carre Yep ey bit ok Bad as aly ite a 40 39° “Ar 
Dog GL = OAAE PD MOAK tea SP Re ee ee 65° 64° Kaa’ 
O Sn CIS TEATS Lae es ce en eae Spice ta 14° 28%4' 
9 f= AIST eAG TR ae Pera ah es Lota eee 78° 78) 5t 
Me. ad 20 GA RS CEST AOE ee ee eee eee eee 19° toe 
ee BEY Bay et Lae tee Corn Tae ee 33° 32> S01 
SO. SSCS The CAT AG Pom Meet ess eT 50° 49° 23’ 
W300 STAR SE TA yaa eee ae ee A ene 16° iG 
The planes of ~ 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 z and w are striated parallel to the adjoining edges of the 
unit rhombohedron, and the planes of the unit rhombohedron and 
the rhombohedron 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 le parallel to the bounding planes of the unit rhombohedron. 
Their character is illustrated in the accompanying figure (Fig. 10). 

Fic. 1o—Growth figures on plane of +R. Calcite, Joplin, Mo. 
