Feb. 1900. Crystal Forms of Calcite — Farrington. 237 



FORMS OF SMALLER CRYSTALS. 



None of the following crystals have the large size of those of the 

 types above described. They show, however, characteristic combi- 

 nations which warrant their description as types. 



Type 3. — Crystals of this type are illustrated in Fig. 3, PI. xxix. 

 As is usual with the Joplin crystals they are not highly modified, 

 being made up solely of the scalenohedrons v, + R3 and w, £ R2. 

 The specimens of these crystals in the Museum collection come 

 from the Pelican mine in Joplin, occurring with sphalerite on the 

 characteristic chert breccia of the region. A large group is shown 

 in the frontispiece. The largest single crystal is not over 4 inches 

 (10 cm.) in length, and from this the crystals grade to a very small 

 size. Measurements of the planes made with the reflecting goniome- 

 ter are as follows: 



Measured. Calculated. 



V : v' =2131:2311= 75° 25' 75 22' 



v : v ▼ =2131 : 3I21 = 35 36' 35 36' 



v: v vi=3~i2i : 321!= 46 53' 47 \ l / 2 ' 



w.w' =3145 : 3415= 50° 58' 49° 23' 



w.-a/v=3i45 : 4135= 15 59' 16 



The most striking characteristic of these crystals is the fact, repre- 

 sented in the figure, that the planes of scalenohedron v are smooth 

 and brilliant, while those of w are rough. This differentiation of 

 planes is constant for all the crystals which have come under the 

 writer's observation. The rough planes do not show definite etch- 

 ing figures, but rather under a lens series of troughs crossing one 

 another and following the lines of the cleavage rhombohedron. In 

 the writer's opinion the roughness is due to irregularities of growth 

 of the crystals rather than to etching. The reasons for this opinion 

 are: First, it is difficult to conceive of any etching agent which 

 could leave the planes of one scalenohedron so entirely untouched 

 while those of the other were so deeply corroded. Second, on sub- 

 mitting the crystal to the action of weak vinegar as an etching agent 

 the planes of the scalenohedron v were affected much more rapidly 

 and noticeably than those of w. The etching produced by the vine- 

 gar on the planes of V was in the forms of troughs or striae follow- 

 ing the lines of cleavage, on 7a there occurred only a slight additional 

 roughening of the planes. It is, therefore, in the writer's opinion, 

 not always correct to speak, as is usually done, of the rough planes 

 as etched planes. They will often be more correctly described by 

 the term imperfect or incomplete planes. 



