60 RECORDS 



dividing plane between the two minerals, it is possible, by 

 slightly raising the objective, to observe a " bright line " on the 

 side of the mineral having the higher index of refraction. 



The main precautions to be observed are, that the cone ot 

 incident light be small, the sections very thin, the cementing 

 material not much lower in refractive index than either of the 

 minerals to be determined, and the plane of contact nearly ver- 

 tical and clear. When the contact plane is much inclined, the 

 method cannot be applied. 



By this method very slight differences in refraction can be dis- 

 tinguished ; as for example, between quartz sections cut parallel 

 and at right angles to the optic axis ; in which the difference 



= 0.009, = 1-553. = I-544- 



Dr. Luquer's paper has been published in the School of Mines 

 Quarterly for January, 1902, pp. 127-133. 



Austin F. Rogers stated that the minerals of the Joplin Dis- 

 trict include sulphur, galena, sphalerite, covellite, greenockite, 

 wurtzite, chalcopyrite, pyrite, marcasite, quartz, cuprite, pyrolu- 

 site, limonite, calcite, dolmite, smithsonite, cerussite, aurichalcite, 

 hydrozincite, malachite, azurite, calamine, muscovite, chryso- 

 colla, allophane, pyromorphite, barite, anglesite, leadhillite, cale- 

 donite, linarite, gypsum, goslarite, chalcanthite, melanterite, 

 copiapite and bitumen, all of which have been found by the 

 writer. 



Lamellar twinning has been observed in galena, the twinning 

 planes being vicinal tetragonal trisoctahedra. Covellite is found 

 replacing sphalerite. Wurtzite occurs in distinct hemimorphic 

 crystals — the first instance of the kind to be reported. Twin 

 crystals of marcasite are common, among them cyclic fivelings. 

 Quartz crystals are rare and small. Calcite presents an in- 

 teresting field for crystallographic study, about twenty-four 

 types, with a total of twenty-nine crystal forms, having been 

 noted. Twinning according to all of the four laws for calcite 

 have been observed. Some distinct crystals of aurichalcite con- 

 firm D'Archiardi's observations that the mineral is monoclinic 



