ART. 3. BLACK SANDS FROM IDABO SHANNON". 6 



over with a good magnet. The magnetic portion is usually largely 

 magnetite, but some ilmenite is commonly magnetic. If platinum i? 

 sought the magnetic portion should be carefully examined, as a part 

 or all of the platinum may be magnetic. The magnetite may also 

 contain many grains of nonmagnetic materials, carried mechani- 

 cally by the cohering particles of magnetite, but these are usually 

 only accidental grains of the minerals most abundant in the non- 

 magnetic residue. Some minerals, not themselves magnetic, are very 

 frequently rendered magnetic by the presence of small magnetite 

 inclusions. The nonmagnetic residue may then be examined with a 

 good lens or with a low to medium power microscope, using incident 

 light from above the stage. A binocular microscope is best, if one 

 is available. Many of the more common minerals may be readily 

 identified by their form and color. Except in unusually coarse or 

 water-worn sands, grains of the constituent minerals can be found 

 which are bounded by bright crystal faces. By taking these up on 

 a wire tipped with wax and tilting them backward and forward 

 or rotating them, so as to observe the reflection of light from the 

 faces, an idea can usually be gained relative to the symmetry of the 

 crystal which may serve to identify the mineral when compared with 

 the crystal figures in a textbook or with the drawings accompany- 

 ing this paper. Gold and the platinum metals are easily recognizable, 

 and their identifications can be confirmed by applying a drop of 

 concentrated nitric acid and observing, through the microscope, 

 whether it has any action. 



Perhaps the greatest difficulty will be found in distinguishing 

 between relatively opaque dark nonmagnetic grains, which may in- 

 clude chromite, ilmenite, limonite, and a number of rarer columbates, 

 tantalates, titanates, etc., and which often are quite devoid of crystal 

 fonn. By carefully transferring any certain grain to a surface of 

 unglazed porcelain and crushing it with a hard object there is ob- 

 tained a powder which, when rubbed fine, may have a distinctive 

 colored streak which will prove of diagnostic value. The presence 

 of uranium or thorium in any appreciable quantity in any of the 

 minerals can be shown by sprinkling some of the sand on a fresh 

 photographic plate which is well wrapped in light-proof paper and 

 allowing it to stand undisturbed for a week or more. When the plate 

 is developed, the presence of radioactive minerals is shown by small 

 clouded exposed spots on the resulting negative varying in intensity 

 with the size of the particle and its degree of radioactivity which 

 is directly proportionate to its content of uranium or thorium. 



Methods of goniometric study on such minute crystals can be 

 applied only by a crystallographer with improved apparatus and to 

 grains or crystals having bright faces. The results are usually con- 

 clusive, but unfortunately this is not invariably the case. In the 



