74 Thirty- first Report on the state Museum. 



favorable case, twelve beams, which cross each other, in the center of the light, 

 at angles of 60 degrees, or if twelve rays appear, at angles of 30 degrees. 



The star thus produced resembles the 6— (—6 diagonals of figure 1, and is based 

 upon their crystallographic direction and value. It will be observed, with 

 twelve-sided stars, that six beams are of greater intensity, whilst the interme- 

 diate ones are less luminous. The capacity of certain mica to present to the 

 eye this star-like figure is called asterism. 



Zirkel, in his famous work, Mikroxkopische Beschafferheit der Minn 'alien 

 unci Gesteine, 1871, reports likewise a phlogopyte of South Burgess, Canada, 

 which shows asterism. Zirkel explains this phenomenon by the presence of a 

 number of microscopical crystals which are disseminated through the mica in 

 directions making with one another an angle of 60 degrees. 



The phlogopytes of the above-named New York State localities, with the 

 exception of that of Monroe, Orange county, present, in unaltered specimens, 

 asterism, but in a less degree than those from South Burgess and Perth, 

 Canada, which latter ones are of an even and smooth surface, whilst the above- 

 named phlogopytes are uneven, bent, and partly altered. 



In a lamina from Pope's Mill's, St. Lawrence county, not larger than one 

 square inch, one part of it presents asterism, while the other altered one does not. 

 It is, however, difficult to determine, with a limited number of specimens of 

 phlogopyte lacking asterism, whether this is to be attributed to a molecular 

 alteration, or whether the phenomenon never has been present even in fresh, 

 unaltered pieces. 



The investigations which the writer has made with various micas, and other 

 minerals, in regard to their asterism, induce him to suggest that asterism is not 

 due to microscopical enclosures, arranged in a hexagonal direction ; but, on the 

 contrary, that the crystals, in the moment of their origin, were compelled, by 

 the crystallographical, molecular arrangement of phlogopyte, to which asterism 

 is due, to place themselves in a manner conforming to the crystalline architec- 

 ture of the mineral. The reasons for this view are : 



1. Some laminae of phlogopyte show asterism in a distinct manner, without 

 having the microscopical crystals. 



2. Some laminae of phlogopyte show asterism in six-sided beams, and contain, 

 also, those microscopical, prismatic crystals, the direction of which is a parallel 

 one 



3. According to Zirkel's explanation, only one beam should be seen, as only 

 one direction of crystals occurs in phlogopyte. 



4. Some altered phlogopyte laminae contain those crystals, nearly unaltered, 

 and no asterism could be seen. 



5. Jeffersite presents asterism in a brillant manner, and has no microscopical 

 enclosures to which this optical quality might be attributed. 



The phenomenon of asterism is best observed with thin laminae, holding them 

 near to the eye, parallel to the source of the light. The size of the laminae 

 should not be too small, as, in the latter case, the lamina has to be brought 

 close to the eye to observe it, which might lead to some deception. The 

 light of a candle is best ; observations should be made in a dark room, where 

 only the candle-light is burning, though asterism will be shown in daylight, but 

 less distinctly. If the laminae (unaltered) are thin, uniform, and of a large sur- 

 face, a twelve-sided star will be seen, its beams being fine, distinct lines, crossing 

 at the centre of the light, and losing their intepsity towards their outer zone. 

 In advancing the phlogopyte toward the light, the rays will appear to pass into 

 broad lines, which taper out towards their less luminous part. The broad 

 beams have bright lines on their borders, whilst the interior is. dark, or trans- 

 versely striated with prismatic colors. If a thicker lamina is used, as a rule, 



