412 Clarke and Schneider — Experiments upon the 



interference colors are near the white of the first order. Ex- 

 tinction takes place strictly parallel to the prismatic surface. 

 Terminal faces rounded, or unequally developed. Beside 

 prisms there are square or rhomboidal folise, probably of the 

 same substance. The inclusions were first observed by G. 

 Rose (Neues Jahrbuch, 1863, p. 91) who regarded them as 

 kyanite. Rosenbusch describes them again, and determines 

 them as tourmaline (Physiog. der Mineralien, p. 486). The 

 prisms certainly correspond well in their optical characteristics 

 to this mineral. Kyanite and apatite are excluded from among 

 the possibilities. Probably, in spite of its apparent abundance 

 the mineral is but a very small fraction of the mica substance. 

 At Mr. Lindgren's suggestion the mica was carefully tested 

 for boron, but none was found. Hence tourmaline, if present, 

 must be in exceedingly small quantities. 



On account of the high proportion of titanium in the Port 

 Henry iron mica, this too was examined by Mr. Lindgren, who 

 reports as follows : " It is a dark-brown, unusually deep colored, 

 apparently uniaxial biotite, without inclusions, and especially, 

 as far as examined, free from any titanium mineral." Hence 

 the titanium is to be regarded as a constituent of the mica itself. 



The action of gaseons hydrochloric acid upon these micas, at 

 383°-412°, was almost insignificant. The data are subjoined. 



Hours heated 



Burgess. 

 12 



Edwards. 

 20 



"Too 



•21 

 •13 



Port Henry. 

 33 



MgO removed 

 R.O, « ;■__. 

 SiO, 



•40 

 none 



trace 

 •44 



In the case of the Port Henry mica some iron was volatilized 

 as chloride. This was estimated; and it was found that the 

 total iron taken out, reckoned as FeO, amounted to only 162 

 per cent. An experiment on the Edwards phlogopite at 498°- 

 527°, lasting 18 hours, gave 1*41 per cent of removable mag- 

 nesia. This quantity has a possible bearing upon the formula 

 of the mineral. 



By aqueous hydrochloric acid all three of the micas were 

 completely decomposed. Hence the Burgess phlogopite could 

 have contained little tourmaline, for that mineral is not soluble 

 in the acid. Moreover, the solubility of the micas prevents 

 us from assuming in either of them an admixture of a musco- 

 vite molecule, for muscovite also is insoluble. A careful com- 

 parison of the two phlogopites showed that the fluoriferous 

 variety was much more stable towards acids than the rarer 

 non-fluoriferous mineral ; a fact which also appears in the 

 action of the gaseous acid upon them. When the two varieties 



