Feb. 1900. Crystal Forms of Calcite — Farrington. 239 



Chicago. The crystal is shpwn in Fig. 1, PI. xxx. Measurements 

 made in the zone of the negative rhombohedrons on this crystal with 

 the contact goniometer are as follows: 



Measured. Calculated. 



^.■77=0112:0775 = 28 27 51' 



77.-/=o775 :o22i = 9° o° I' 



e : /=OM2 :022I = 36 36° 52 



e : 2"=oi 12 : 0.1 1. T i.i= 59K" 58° 29 



The determination of JS" rests rather on its occurrence on other crys- 

 tals of this type which could be measured with a reflecting goniome- 

 ter, than on the value obtained on this crystal, it being presumed 

 that the same plane was most likely to occur again. 



The peculiar distortions and striations exhibited in this crystal 

 are illustrated in the figure. How far these have caused it to depart 

 from the normal form may be seen by comparing Figs, i and 2, PI. 

 xxx. The tendency shown in this crystal for the two individuals of 

 the twin to grow by one another and partially complete themselves, 

 is quite common among the Joplin twins. Further struggles of the 

 molecular forces are illustrated by the number of abortive planes of 

 e, -^R, these being so numerous as to produce cross striations on 

 /, -2R. The latter planes are further rounded from the tendency to 

 produce scalenohedral forms. 



Another and the only other type of twin crystal noted has e, 

 -J^R as the twinning plane. Such twins occur among a group of 

 calcite crystals formed together on a specimen in the collection of the 

 University of Chicago. The crystals of this group are in general 

 scalenohedral in form, colorless to amber in color, and vary from 

 1 to 2 inches (.3 to .6 cm.) in length. Many are simple crystals 

 made up of v, +R3 dominant, modified by e, -^R, /, -2R, and a 

 form new to calcite, fl, -20R. * Others are intermediate in charac- 

 ter, and are the result, doubtless, of one individual pushing by 

 the other, as twins on the base have already been shown to do. Such 

 a crystal is represented in Fig. 1, PI. xxxi, the crystal being drawn 

 in the position of the negative scalenohedron in order better to show 

 the elongation in one direction. This elongation characterizes the 

 true twins as well. In this crystal on the side toward the observer, 



♦The letter given to this form was chosen in accordance with Goldschmidt's system, Dr 

 Charles Palache having kindly indicated to me what the appropriate letter, according to this sys- 

 tem, would bo. I do not feel called upon to apologue forthus combining Oana'sand Goldschmidt's 

 lettering, f deem it essential that new forms should be lettered in accordance with Goldschmidt's 

 system in order to avoid conflict of letters, but the common forms are, and are long likely to be, 

 better known by Dana's letters. 



