220 



EEPOBT OF NATIONAL MUSEUM, 1899. 



Nos. 73253, 73254, U.S.N.M.). Others of less purity occur near 

 South Framingham, Massachusetts (Specimens Nos. 62767, 62768, 

 U.S.N.M.), Lake Umbagog, New Hampshire (Specimen No. 29322, 

 ILS.N.M.), at White Head Lake, Herkimer County, New York (Spec- 

 imen No. 62913, U.S.N.M.), and at Grand Manan, New Brunswick 

 (Specimen No. 57339, U.S.N.M.). 



Chemical Composition. As already intimated, this earth is of a 

 siliceous nature, and samples from widely separated localities show 

 remarkable uniformity in composition. Of the following analyses, 

 No. 1 is from Lake Umbagog, New Hampshire, No. II, from Morris 

 County, New Jersey, and No. HI, from Popes Creek, in Maryland. As 

 will be noted, the silica percentage is nearly the same in all. 



The substance may therefore be regarded as a variety of opal. 



Uses. The main use of infusorial earth is for a polishing powder. 

 It is, however, an excellent absorbent, and has been utilized to mix 

 with nitroglycerine in the manufacture of dynamite. It has also been 

 used to some extent in the preparation of the soluble silicate known 

 as water glass. The demand for the material is therefore quite smal 1 , 

 not nearly equal to the supply. The Maryland and Nevada deposits 

 are said to be the principal ones now worked. During the year 1897 

 the entire output was about 3,000 tons, valued at some $30,400. 



2. CORUNDUM AND EMERY. 



CORUNDUM. Composition, sesquioxide of aluminum A1 2 O 3 , = oxygen, 

 47.1 per cent; aluminum, 52.9 per cent. In crystals often quite pure, 

 but frequently occurring associated in crystalline granular masses with 

 magnetic iron, and often more or less altered into a series of hydrated 

 aluminous compounds, as darnourite (Specimen No. 82492, U.S.N.M.). 

 The crystalline form of the mineral is hexagonal, or sixsided in out- 

 line, and often with curved sides and square terminations, giving rise 

 to roughly barrel-shaped forms, as shown in specimen No. 81450 from 

 Bengal, India. 



A prominent basal cleavage causes the crystals to break readily with 

 smooth, flat surfaces at right angles with the axis of elongation. The 

 massive forms often show a nearly rectangular parting or pseudo- 



