SOMERS: MICROSCOPIC EXAMINATION OF ClyAYS 121 



it at once. More generally, however, it is found upon close 

 examination as very minute grains or needles which are never- 

 theless so clear-cut that their refractions can be plainly seen. 

 The interference color is of the first order, but the particles are 

 so small that the actual birefringence is thereby shown to be 

 very high. They vary from perhaps cooi mm. to o.oio mm. in 

 diameter and if in needles are 5 or 6 times as long as they are 

 wide. In number these grains are very abundant, but in actual 

 bulk, they represent a very small quantity of rutile. 



Rarer Minerals. — Tourmaline is well marked by its pleochrom- 

 ism, and its frequent occurrence is notable. Epidote is occa- 

 sionally seen as a slightly greenish mineral of moderate single 

 and double refractions. Grains of both high index and bire- 

 fringence are common, though not in any abundance, and they 

 prove to be zircon and titanite. A distinction between the two 

 is quite possible by means of the higher interference color of 

 the titanite. Zircon is much the commoner. 



Diaspore. — This is easy to determine by its moderately high 

 index and double refraction, and its occurrence in irregular 

 grains. It is quite common in certain of the clays examined 

 from Missouri. Since the diaspore in these clays or their as- 

 sociated rocks sometimes occurs in grains sufficiently large to 

 be seen with the naked eye, it may be mistaken for quartz, but 

 can be separated from it and clay by means of bromoform (sp. 

 gr. 2.8), in which the diaspore (sp. gr. 3.4) sinks, while the as- 

 sociated minerals float. ^ 



Texture. — It would be very difficult to standardize clays by 

 size of particle, because any one clay is apt to be made up of 

 particles of all sizes, and because there is no standard to use. 

 Comparing these clays with each other, however, it may be said 

 that relatively coarse grains average o. 100 mm. or more in size, 

 medium grains 0.020-0.025 mm., and fine grains o.oio mm. or 

 less. 



MINERALS IN THE BURNED CEAY 



A number of the clays were molded into one-inch cubes. 

 These were all fired for 8 hours up to 950° C. After this one 



* Wherry, E. T. Field identification of diaspore. Amer. Mineral. 3: 154. 1918. 



