
a 
Fes. 1900. Crystal Forms or CaLcIrE—FARRINGTON. 239 
Chicago. The crystal is shown in Fig. 1, Pl. xxx. Measurements 
made in the zone of the negative rhombohedrons on this crystal with 
the contact goniometer are as follows: 
Measured, Calculated. 
é: IT=o112 :0775 ate MR. «As ae, ee Ae oe oy whe 
I > f=0775 : 0221 = Sy Sete hace ntact OF a 9° "aay oe 
ME PRDERO S26 100 i ie Ns Ke vc ood Gaeeea 36° 36° 52° 
LST ES A ye Ca etre a an RP oe Se 59%° Aekee seh 
The determination of > rests rather on its occurrence on other crys- 
tals of this type which could be measured with a reflecting goniome- 
ter, than onthe 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. 1 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 
f, -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, 
—¥%4R 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 
I to 2 inches (.3to.6cm.) in length. Many are simple crystals 
made up of v, +R3 dominant, modified by e, -%R, f, -2R, and a 
form new to calcite, Q, —-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, Pl. 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. Inthis 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 be. I do not feel called upon to apologize for thus combining Dana’s and Goldschmidt’s 
lettering. I 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. 
