78 



Thtrty-first Report ox the State Museum. 



Another form of microlites, in form of laminae, and most likely a mica, is pre- 

 sented and best viewed under the dark field of the crossed Nicols. These mica 

 laminae are likewise spread through the hematite microlites, and seCm to form 

 one crystal, but on account of their vivid polarization in azure-blue and yel- 

 lowish colors, are easily separated from them. 



Most of the minute mica scales present lines of twin-formation in such a sharp 

 and well-defined manner, that they resemble the plagioclose-lines (lines of twin- 

 formation), under the polariscope, and cause some doubt in regard to their being 

 mica. 



In most of the preparations of Pope's Mills and Edwards phlogopyte, the 

 bronze color of the base is due to the above-described mica-hematite microlites ; 

 in all those sections seeli, a want of the long band-shaped mica crystals is char- 

 acteristic. They may occur in a few crystals, but polarize less intensely than 

 those which occur in phlogopyte, the base of which is uniformly colored by a 

 brownish-yellow coloring-matter. 



The decomposition of the phlogopyte of Pope's Mills seems to be due to the 

 presence Of the mica-hematite microlites. They change into a hydrated oxide of 

 iron, which becomes dissolved and carried away by water, which may come in 

 contact with it. 



[Note. — The above paper is published from manuscript communicated by 

 the author, without the proof-sheets having been submitted to him, owing to his 

 present residence in Honduras. The accompanying plate is given as originally 

 sketched by him, containing some unnumbered figures not referred to in the 

 text.] 



