34 Field Columbian Museum — Geology, Vol. III. 



which did not appear as cracks upon the surface of the section. The 

 extinction was parallel to the principal cleavage, which lies in the 

 plane of the axis of least elasticity. The index of refraction of this 

 mineral was greater than that of the quartz. The cement throughout 

 the entire slide was part of one crystal with the growth of which the 

 sand grains present had not interfered. This was indicated by the 

 cleavage, which was everywhere parallel with itself, and by the 

 interference color which was the same hroughout the slide. The 

 high specific gravity of the specimen and the presence of much barium 

 sulphate, taken with the features shown in the slide indicate that this 

 cement is barite. In this slide it was evidently cut parallel to m and 

 showed the usual cleavage parallel to c and one set parallel to m. 

 An analysis of the specimens made in the Museum - laboratories 

 by the author gives the following result : 



Si0 2 36 . 99 



BaO 35.76 



S0 3 19. 20 



Fe 2 0, 0.82 



Al 2 O a 5-36 



CaO 0.51 



MgO 0.03 



H 2 0* ' , 0.27 



Organicf 0.32 



99.26 

 This corresponds with a mineral composition (disregarding the 

 silica required for the aluminous minerals) of: 



Barite ' 54.42 



Quartz 36.99 



Miscellaneous 8.59 



100 .00 

 From the analysis it would appear that some aluminous mineral 

 is present bu ; the slides fail to disclose such in quantities required to 

 satisfy the analysis. Inasmuch as barite frequently contains 

 similar elements as' mpurities even when well crystallized, it appears 

 best to provisionally include the minor elements in the barite for an 

 approximate determination of mineral composition. The mineral 

 composition thus becomes: 



Barite 63 



Quartz 37 



100 

 This corresponds to a specific gravity of 3.77 against 3.380 



* From air-dried specimen, by Penfield's method, 

 t Loss on ignition less water. 



