■* PROCEEDINGS OF THE ACADEMY OF 



I^otes on MICBOSCOPIC CRYSTALS included in some Minerals. 

 BY ISAAC LEA. 



During some j^ears past I have given much attention to the examination of 

 minerals under the microscope, and some of the observations were published 

 in the Proceedings of the Academy in 1866. 



About a jear since, in the examination of a thin fractured piece of a large 

 garnet from North Carolina, I was surprised to observe a number of very 

 minute acicular crystals, which generally took two or three directions. This 

 induced me to examine more closely into the varieties of garnets which were 

 accessible to me, and supposing these crystals might have been observed by 

 others, I referred to the principal works on mineralogy which have been pub- 

 lished in France, Germany and in this country. 



In none of these have I found any mention of these inclusions. But in that 

 excellent work "Repertoire D'Optique Moderne," by M. I'Abbe Moigno, where 

 he treats of optical mineralogy, I found that he states IM. Babinet to have ex- 

 amined ''■ star garnets" [Granats asteriques) some with four and some with six 

 branches. He says that the star garnets with four branches are not very rare, 

 — 20 to 30 in 1000 to 1200— but that the star of six rays he found only one in 

 6000 specimens. Whether the yi7rtwe?iM or _^ie7-«, as M. Babinet callsthe aste- 

 roid reflections, are the same as the acicular crystals observed by me 1 cannot 

 say, but certainly these latter are more common so far as my observation has 

 extended, and 1 have observed no asterisms whatever. 



In 154 specimens of Bohemian polished garnets, 1 found 48 with acicular 

 crystals! This far exceeds the proportion stated by M. Babinet. 



In the precious garnet from Green's Creek, Delaware Co., Penn., (uncut 

 specimens), I found in the close examination of 310 specimens that 75 were 

 possessed of acicular crystals, being nearly 25 per cent. — a very much larger 

 percentage than mentioned by M. Babinet. Of the Brazilian Pyrope I examined 

 40 specimens. They were very pure and free from spots and cavities. I could 

 not find a single acicular crystal in any one of them. 



In Essonite I found no acicular crystals in the few specimens which I had it 

 in my power to examine, nor in grossularite, ouvarovite, colophonite or massive 

 magnesium garnet. 



C'nnamon-stone from Dixon's, near Wilmington, Del., was carefully examined 

 in nearly 60 specimens, none of which showed any trace of acicular crystaliza- 

 tiou. 



Spinelle ruhg, of which I examined 28 specimens, produced no microscopic 

 crvstalized forms. 



It will be difficult to ascertain what composes these microscopic crystals in 

 garnets, but they may prove to be rutile when chemical analysis shall 

 l»e able to resolve the difficulty. 



3Iarch 2d. 

 The President, Dr. Hays, in the Chair. 

 Thirty members present. 



71/arc/i 9th. 

 The President, Dr. Hays, in the Chair. 

 Thirty five members i:)resent. 



The Iblk^wing papers were presented for publication : 

 Third Contribution to tlie Fauna of the Miocene Period of the 

 United States. By Edw. D. Cope. 



[March, 



