THE NONMETALLIC MINERALS. 337 



The name fullers' earth (Walkerde, volaorde, terre a foulon, terra 

 da purgatori, etc.) includes a variety of clay of a greenish white, 

 greenish gray, olive and oil green or brownish color, very soft, with a 

 greasy feeling. It falls into powder in water, imparts a milky hue to 

 the liquid, and appears to melt on the tongue like butter. It was for- 

 merly used by fullers to take the grease out of cloth; hence the name. 



The English beds, according to Geikie 1 occur in Jurassic and Cre- 

 taceous formations. Fullers' earth from beds at Nutfield, near Red- 

 hill, Surrey, England, is described 2 as a heavy blue or yellow clay, 

 with a greasy feel and an earthy fracture. 



When examined with a microscope it is found to consist of 

 extremely irregular corroded particles of a siliceous mineral which in 

 its least altered state is colorless, but which in nearly every case has 

 undergone a chloritic or talcose alteration whereby the particles are 

 inverted into a faintly yellowish green product almost wholly on polar- 

 ized light. The particles are of all sizes up to 0.07 mm. The larger 

 portion of the material is made up of particles fairly uniform in size 

 and about the dimensions mentioned. In addition to these are minute 

 colorless fragments down to sizes 0.01 mm. and even smaller. 



The minute size of these colorless particles renders a determination 

 of their mineral nature practically impossible. But the outline of the 

 cleavage flakes is evidently suggestive of a soda lime feldspar. The 

 high percentage of silica in the insoluble residue would indicate the 

 presence of a considerable amount of free quartz. This, however, the 

 microscope only partially substantiates, very few of the particles 

 showing the brilliant polarization colors characteristic of this mineral. 



When the powder is treated with hydrofluorsilicic acid it yields 

 abundant crystals of potassium and aluminum fluosilicate, together 

 with radiating forms of calcium fluosilicate. The material differs 

 from that last described in that its particles are much larger and more 

 angular in outline and the various elements in a different state of com- 

 bination. (See Plate 17, fig. 2.) 



A substance recently put upon the American market as a fullers' earth 

 (Specimen No. 62737, U.S.N.M., from Enid, Oklahoma), under the 

 trade name of "glacialite," has the following chemical composition, the 

 material being dried at 100 C. before analyzing: 



Silica 50. 36 



Alumina 33. 38 



Ferric oxide 3. 31 



Sodium, lithium, potassium oxide 88 



Water 12. 05 



Organic matter Trace. 



Titanium . . . . Trace. 



99.98 



*Text book of Geology. 3d. ed. p. 133. 2 Geological Magazine, VI, 1889, p. 4- KC 



'NAT MUS 99 22 



