228 Pen field and Ford — Calaverite. 



figure 30, page 242. Having in nearly every instance a dis- 

 torted crystal to work with, showing few zonal relations and 

 no clino-pinacoid, and not being able to make measurements 

 from easily identified forms in the striated zone, it soon became 

 evident that the identification of the small terminal faces with 

 a one-circle goniometer would prove a most laborious under- 

 taking. With a two-circle goniometer, on the other hand, both 

 the measurement of the crystals and the interpretation of the 

 results are very much simplified; in fact, without this instru- 

 ment it would have been next to impossible to have identified 

 the forms of some of the crystals. The crystals were orientated 

 on the two-circle goniometer with the striated zone (the ortho- 

 axis of the crystals) parallel with the axis of the vertical circle. 

 Having adjusted so that a measurement between two m faces 

 was obtained by turning the horizontal circle, the position 

 half way between the two m's was taken as the theoretical 

 clino-pinacoid (010), or polar face, while the reading of the 

 vertical circle at this point gave the position of a theoretical 

 ortho-pinacoid (100) in the striated zone. The two readings 

 served as starting points for making all measurements. The 

 clino-pinacoid was observed on but few crystals and the ortho- 

 pinacoid on only one, therefore nearly all of the measurements 

 which will be recorded were made, not from actual faces, but 

 from the theoretical positions of the (100) and (010) forms. 



One peculiarity to be borne in mind in studying crystals of 

 the monoclinic system is that the distribution of the faces at 

 opposite extremities of the ortho-axis have a right- and a left- 

 handed relation, as will be indicated by the figures. The two 

 ends cannot be interchanged. 



As has been stated already, the faces in the zone parallel to 

 the ortho-axis are striated, and they oscillate with one another 

 to such an extent that most of the crystals have very irregular 

 cross sections when viewed in the direction of their longer- or 

 ortho-axis. In representing the crystals, orthographic projec- 

 tions upon the clino-pinacoid have been employed, and the 

 figures are intended to show irregularities of contour, as well 

 as variations in the relative size and distribution of the termi- 

 nal faces as exhibited by the specimens. 



In order to shorten and simplify the description of the crys- 

 tals they will be treated in groups, as far as possible, and the 

 symbols of the forms together with their measured and calcu- 

 lated angles will be condensed in tables. It would consume 

 altogether too much space and be of too little interest to give 

 separate tables of measured and calculated angles for each crys- 

 tal which has been examined. 



Occurrence Wo. 1. Crystals from the Monument Mine. — 

 The material was presented to us by Mr. Cahn, and consists of 





