Penfield and Sperry — Composition of Howlite. 221 



microscope were resolved into flattened prismatic forms, having 

 somewhat the shape of thin stilbite crystals, the largest 0*23 X 

 0*27 mm., usually broken at the extremities but occasionally 

 terminated by two dome faces. In polarized light they showed 

 parallel extinction, brilliant polarization colors, red and yellow 

 of the first order, according to the thickness of the crystal, the 

 longer axis being the axis of least elasticity. In convergent 

 light an obtuse bisectrix was very indistinctly seen, the plane 

 of the optic axes being at right angles to the longer axis of the 

 crystal. The crystallization is therefore probably orthorhombic. 

 Owing to the slight solubility of the mineral in water its speci- 

 fic gravity was taken in alcohol and found to be 2*59 referred 

 to water. A sample was taken for analysis by breaking into 

 the nodule and carefully freeing from adhering gypsum by 

 hand picking, but it was impossible to obtain the borate per- 

 fectly pure, a little S0 8 being found on analysis which 

 undoubtedly resulted from adhering gypsum. The results of 

 analysis of tbe air-dry powder are as follows : 



I. II. Mean. 



Si0 2 14-65 14-74 14-70 



B 2 0,_ 42-68 42-70 42-69 



CaO 28-22 28-19 28-20 



Na 2 0._ -55 -47 -51 



K 2 -12 -12 



H 2 12-01 11-94 11-97 



S0 3 1-93 2-10 2-01 



100-20 

 Below the results are given after deducting 4*32 per cent of 

 gypsum corresponding to 2 - 01 per cent of SO a , and calculating 

 to 100*00 per cent together with the analysis of How and the 

 theoretical percentages derived from our ratio. 



Calculated for 



H 5 Ca 2 B 5 Si0 14 . 



15-31 



44-65 



28-56 



11-48 





Ratio. 





How. 



Si0 2 15-33 



•255 



2-00 



15 "25 



B 2 3 44-52 



•636 



5-00 



[44-22] 



CaO 27-94 



•498 ) 





28-65 



Na 2 0_ .. -53 



•008 J 



•507 3-99 





K 2 0.... -13 



•001 ) 







H 2 0___. 11-55 



•641 



5-04 



11-84 



100-00 100-00 100-00 



The ratio of Si0 2 : B a O, : CaO : H 2 = 2:5:4:5 very closely, 

 with a small part of the CaO replaced by Na 2 and K s O. This 

 is exactly the ratio obtained by Prof. How, but the analysis is 

 of importance as proving the identity of this curious boro-sili- 

 cate as a distinct species, the original analysis having been 

 made on dense chalk-like varieties and the most important con- 



