218 REPORT OF NATIONAL MUSEUM, 1899. 



The tripoli is everywhere underlain by a relatively thin bed of 

 stiff red clay, and also traversed in every direction by seams of the 

 same material from 1 to 2 inches thick. These seams and other joints 

 divide the rock into masses which vary in size up to 30 inches or more 

 in diameter. Microscopic examinations as given by Hovey show the 

 rock to contain .no traces of organic remains, but to be made up of 

 faintly doubly refracting chalcedonic particles from 0.01 to 0.03 milli- 

 metre in diameter. The chemical composition, as shown from analysis 

 by Prof. W. H. Seaman, is as follows: 



Silica (Si0 2 ) 98.100 



Alumina (A1 2 3 ) 0-240 



Iron oxide (Fe O and Fe 2 O s ) 0. 270 



Lime (CaO) 0.184 



Soda (Na,O) 0.230 



Water (ignition) 1. 160 



Organic matter 0. 008 



100. 192 



Silica soluble in a 10 per cent solution of caustic soda on boiling three hours, 7.28 

 per cent. 



Aside from its use as a filter (Specimens Nos. 62044 and 62045, 

 U.S.N.M.) the rock is crushed between burr stones, bolted, and used 

 as a polishing powder (Specimens Nos. 51231 and 55029, U.S.N.M.). 

 To a small extent it has been used in the form of thin slabs for blotting 

 purposes, for which it answers admirably owing to its high absorptive 

 property, but is somewhat objectionable on account of its dusty char- 

 acter. The view (Plate 6) shows the character of a quarry of this 

 material as now worked by the American Tripoli Company at Seneca, 

 in Newton County. 



DIATOMACEOUS OR INFUSORIAL EARTH, as it is sometimes called, 

 is, when pure, a soft, pulverulent material, somewhat resembling 

 chalk or kaolin in its physical properties, and of a white or yellow- 

 ish or gray color. Chemically it is a variety of opal (see analyses 

 on page 220). 



Origin and occurrence of deposits. Certain aquatic forms of plant 

 life known as diatoms, which are of microscopic dimensions only, have 

 the power of secreting silica, in the same manner as mollusks secrete 

 carbonate of lime, forming thus their tests or shells. On the death of 

 the plant the siliceous tests are left to accumulate on the bottom of 

 the lakes, ponds, and pools in which they lived, forming in time beds 

 of very considerable thickness, which, however, when compared with 

 other rocks of the earth's crust are really of very insignificant propor- 

 tions. Like many other low organisms the diatoms can adapt them- 

 selves to a wide range of conditions. They are wholly aquatic, but 

 live in salt and fresh water and under widely varying conditions of 



