288 O. C. Farrington — Datolite from Guanajuato. 



Another interesting zone well developed on this crystal is that 

 of x over t to x. In this zone the measured and calculated angles 

 compared as follows : 





Measured. 



Calculated 



102^320 



49° 20' 



49°21' 



320^111 



24 46 



24 46 



111^013 



56 45 



56 40 



013^102 



49 15 



49 13 



In crystal ~No. 3, shown in fig. 4, the forms are much 

 extended in the direction of the clinodiagonal axis, making a 

 thicker crystal and one which displays more than the others 

 the rhombohedral appearance noted by Dana. Here the pyra- 

 mids and prisms appear only as a slight truncation of the edge 

 formed by the meeting of the orthodome and the clinodomes, 

 and might therefore easily be overlooked. The crystal shows no 

 new faces except the positive hemi-orthodome 2, 302, which 

 here appears for the first time. 



The crystal shown in fig. 5 is peculiar for displaying a mero- 

 hedrism worthy of note. The series of fundamental positive 

 he mi-pyramids is here represented in the upper rear octants by 

 e, 112, X, 113, (this face not being wanting from the series as in 

 the other crystals) and k, 114. These faces are all small, and 

 only slightly truncate the edge formed by the junction of 

 the orthodome x and clinodome t. In the lower front octants, 

 however, X and k are wanting and only e appears, so greatly 

 enlarged that it is equal to t in size. The crystal shows, there- 

 fore, inclined-faced hemihedrism with respect to X and k, and 

 were e not present could properly be termed hemihedral. As 

 this face is present, however, the absence of the other faces 

 should more properly perhaps be regarded as accidental and 

 the occurrence be considered due to merohedrism. 



It may be worthy of note that in measuring the smaller faces 

 I found direct sunlight reflected by a movable mirror a more 

 satisfactory source of light than any artificial light. Signals 

 could be obtained from faces by this means which gave none 

 whatever with artificial light. With a darkening attachment 

 on the goniometer work in a dark room with its heat and bad 

 ventilation is thus avoided and in most cases good readings 

 could be made from sunlight even without the darkening 

 attachment. 



Field Columbian Museum, Chicago, Feb. 1, 1898. 



