436 PROCEEDINGS OF THE AMERICAN ACADEMY. 



anglesite is the highly perfect basal cleavage parallel to which the lustre 

 is pearly. The crystals detached for measurement are with one excep- 

 tion minute fragments removed from aggregates or larger crystals ; the 

 cleavage develops so readily that it is exceedingly difficult to remove a 

 crystal entire. These fragments are in nearly all cases, therefore, 

 bounded by cleavage above and below, with edges more or less com- 

 pletely faceted with faces of pyramids, domes, and prisms. Their com- 

 plex character may be judged by one crystal (Table II, No. 14, p. 439), 

 a fragment about 2 mm. in diameter, on which were measured seventy 

 faces belonging to thirty-five forms. On this crystal and some others, 

 faces of both positive and negative forms occur on the upper end of 

 the crystal ; in others the forms are clustered about the end of the a 

 axis, so that the positive forms are on the upper part and negative ones 

 on the lower part, requiring two adjustments on the goniometer for 

 measurement. With added complications due to twinning, described 

 in another place, the adjustment of the crystals, their orientation, and 

 the interpretation of the forms, were problems of some difficulty, which 

 could hardly have been solved without the use of the two-circle goniom- 

 eter and of the graphical method in gnomonic projection. The method 

 followed was generally as follows. The basal cleavage, always present, 

 is so nearly in polar position (/3 = 89° 30'), that an approximate adjust- 

 ment was made by its means. The prism zone was then sought by 

 turning the horizontal circle of the goniometer 90° from polar position, 

 and this zone if present gave a final adjustment. In some cases it was 

 necessary to make a rough determination of some of the forms with the 

 first approximate adjustment by the base, and then to readjust to the 

 calculated angles of these forms, a somewhat laborious but entirely 

 accurate process. 



Once adjusted, the clinodome zone could generally be recognized by 

 its striated character, but in general no attempt to identify the forms 

 was made until a projection had been constructed from the measure- 

 ments. Here the principal zones at once appeared, and the positive 

 and negative forms could be separated and forms in twin position 

 sifted out. Cases were very rare where by these means the orientation 

 of the crystal could not be made with entire certainty. 



Some twenty crystals were measured, and of these fifteen yielded 

 measurements that could be used in the computation of the elements. 

 Sixty-three forms were observed, as shown in Table I, in which is given 

 for each the computed angles <f> and p, the arithmetic mean of the ob- 

 served values of </> and p, the deviation in minutes of the extreme 

 observations for each from the computed value, and the number and 

 quality of the observations. 



