MEMOIR ON EMERY. 27 



those of Gumuch-dagh and Manser in Asia Minor, and the 

 islands of Naxos, Samos, and Nicaria in the Grecian Archi- 

 pelago ; and there is reason to believe that this mineral will be 

 found in almost every corundum locality. I have already 

 found it on crystals of corundum from. China. 



In examining the emery formations one of the first things 

 that struck my attention was the existence of diaspore and 

 corundum together, then observed for the first time. The same 

 year M. Marignac discovered it in the limestone of St. Gothard, 

 along with the well-known crystals of corundum that exist 

 there. Having found the diaspore under these new circum- 

 stances, it has been examined with much attention. 



At Gumuch-dagh the diaspore is found in flattened and 

 rounded prisms, with the surface streaked with lines that afford 

 by reflected light an iridescence. Crystals with perfect sum- 

 mits are rarely found, and during two or three days' examina- 

 tion on the place I found only five small crystals with one of 

 the summits perfect; they were, however, very beautiful, and 

 finer probably than any yet known. Not wishing to lose so 

 favorable an occasion to verify the crystallography of diaspore, 

 I requested M. Dufrenoy to undertake the measurement of the 

 angles, and it is to this able professor that we are indebted for 

 the crystallographic results here given. * 



The crystals are elongated needles crossing each other in 

 all directions, like an acicular variety of aragonite from the 

 Yosges. They resemble small crystals of topaz in luster and 

 in the disposition of the vertical striae on the faces g. Their 

 color is yellowish- white. They are strongly dichroitic ; the 

 summits under certain inclinations appear black, as if the light 

 was completely polarized. The cleavage is very easy parallel 

 to the face g l , and it is this cleavage that gives a lamellar 

 structure to that diaspore which is not in the form of needles. 

 This cleavage, notwithstanding its facility, does not expose 

 surfaces that reflect with great accuracy ; it is the only angle 

 which offers the difference of a half degree ; repeated measure- 

 ments of the other angles never varied more than four minutes. 

 The pearly luster of the cleavage in connection with its striated 



* Three of the crystals measured are in the Cabinet of the School of Mines 

 and Garden of Plants at Paris. The second crystal above is nearly as thin 

 as the first, although represented thicker, in order to show well all the planes. 



