238 MIATtGYRITE. 



sessing a more distinctly ciystalline struc- 

 ture, and a less amount of magnesia in their 

 composition than those of the first-mentioned 

 class. 



Prof. Lawrence Smith states that all the 

 meteoric stones found in America contain 

 Schreibersite, a mineral not known on the 

 earth as a natural compound. 



Various theories have been invented to 

 account for the origin of these stones. By 

 Laplace and others they Avere supposed to 

 be thrown from the volcanos of the moon — 

 by some they are believed to be formed in 

 the atmosphere ; while Chladni and others 

 consider them to be cosmical bodies circu- 

 lating in free space, and occasionally 'at- 

 tracted to the surface of the earth when 

 they come within the influence of its gra- 

 vitation. 



The fall of these stones, according to 

 Prof. C. U. Shepard, is chiefly confined to 

 two zones ; one in America, \y\x\^ between 

 33° and 44° N. lat. and about 25° in length ; 

 and the other in the eastern continent, 

 bounded by the same degrees of latitude, 

 but extending 10° further north, and with 

 twice the length of the American zone. The 

 direction of both these zones is more or less 

 from north-east to south-west. 



The most remarkable masses of Meteoric 

 Iron are, that found in 1783 by Don Rubin 

 de Cells, in Tucuman, in South America, 

 weighing 15 tons; that discovered in 1784 

 on the Riacho de Bendego, in Brazil, esti- 

 mated to measure 32 cubic feet and to weigh 

 17,300 lbs. ; and that on the Red River, in 

 Louisiana, weighing above 3000 lbs., and 

 presenting distinct octahedral crystals. 



A very "interesting suite of meteorites will 

 be found in Cases 1, 1 A, and 1* in the British 

 Museum. 



MiARGYRiTE, H. Rose. Obtiquc : primary 

 form an oblique rhombic prism. Occurs in 

 thick tabular crystals with the lateral planes 

 deeply striated. Colour varying from dark 

 leacl-grey to iron-black. Lustre between 

 metallic and adamantine. Opaque : in thin 

 fragments deep blood-red by transmitted 

 light. Streak dark cherry-red. Soft and 

 easily cut. Fracture subconchoidal. H. 2 

 to 2-5. S.G. 5-3. 



Fig, 288. 



Comp. Ag,S + Sb2,S5 = silver 35-9, anti- 

 mony 42-9, sulphur 21'2 = 100. 



MICA. 



Analysis^ by H. Rose : 

 Antimony . 

 Silver . 

 Copper 

 Iron 

 Sulphur 



39-14 



36-40 

 1-06 

 0-62 



21-95 



99-17 



BB fuses readily : on charcoal with carbo- 

 nate of soda ultimately yields a globule of 

 silver. 



Locality. Braiinsdorf, near Freiberg, in 

 Saxony, associated with argentiferous Ar- 

 senical Pyrites. 



Name. From M-itcav, Jess, and agyy's?, silver ; 

 from its containing less silver than some of 

 the other allied ores. 



Brit. Mus., Case 11. 



MiAsciTE or MiASKiTE. The mineral de- 

 scribed by Dufrenoy under this name is, 

 apparently, pseudomorphous Tremolite in- 

 crusted with carbonate of lime and mag- 

 nesia. It is called Miaskite from its oc- 

 currence in the hills in the neighbourhood 

 of Miask in Siberia. 



Mica, HaUy, Phillips, Allan. Under this 

 term are included several varieties of a 

 mineral which possess in common a flaky 

 structure and a glistening metallic lustre, 

 characters Avith- which most persons are 

 familiar in the shining spangles which form 

 one of the common constituents of granite. 



The Micas are mostly biaxial, and, by the 

 researches of Senarmont, Silliman, Blake, 

 Dana and others, they have been divided 

 into three groups, according to the variation 

 between the apparent inclination of the 

 optical axes; viz. 1. into 3Iuscovites, in 

 which the apparent inclination of the axes 

 varies between 44° and 75° ; 2. Phlagopites, 

 between 5° and 20° ; and 3. Biotifes, in 

 Avhich it ranges below 5°. Lepidomelane is 

 probably uniaxial. 



According to Silliman Jr., the Muscovites 

 are confined to granitic and other igneous 

 rocks, and contain in general potash or 

 lithia, and a small amount of magnesia; 

 while the Phlogopites are only found in 

 granular limestone and serpentine, and con- 

 tain magnesia, and often only a small 

 quantity of alkali. 



From the researches of Schaffhautl, it 

 appears that the composition of the Micas 



289. 



ranges between M 2Siand M^ 3Si, where M 

 (which has a great range of variation) 



