1892.] NEW-YORK MICROSCOPICAL SOCIETY. 63 



red-hot, the crystals and spherules of pyrite are turned to a red 

 oxide soluble in water. Grains not soluble are attracted by the 

 magnet, while as unburned crystals of pyrite the magnet has no 

 influence on them. 



" Four slides to illustrate and place on record for the first time 

 the occurrence of a new deposit of tripoli, being a marine fossil 

 sedimentary rock recently discovered by me, its true character 

 having been misunderstood until I demonstrated it by micro- 

 scopic analysis. Its geographical position is indicated as follows : 

 It occurs at the Big McGrew's Shoal on the Tombigbee 

 River, Clarke Co., Ala., one mile by land northeasterly from 

 St. Stephens, or three miles by river. This shoal is now being 

 blasted out and improved under Government supervision, and 

 specimens of the different locks forming the obstruction were 

 forwarded to U. S. Engineer's office at Mobile, where I casually 

 found it. At this time it seems to have an unique interest, as I 

 know of no other rock or tripoli similar to it in mineral or fossil 

 composition. Its uniqueness lies in the fact that it is a tripoli 

 that breaks down easily to a mud in water, and is very rich 

 in marine fossil Diatoms, Polycistina, Foraminifera, sponge 

 spicules and gemmules, plant debris, and mineral grains, not- 

 ably crystalline chloritic grains ; and that the Diatoms, Poly- 

 cistina, and Foraminifera are infiltrated by transparent mineral, 

 decussating, crystalline plates, not readily soluble in acids, 

 thus nearly obliterating the sculptural markings on their sur- 

 faces, and thus practically being petrified Diatoms, Polycistina, 

 and Foraminifera. The slides sent were made by rubbing the 

 dry tripoli powder, after cold acid treatment, on chamois skin, 

 to remove the undesirable amorphous particles of silica. The 

 Diatoms, Polycistina, and Foraminifera survived this polishing 

 ordeal, thus attesting their toughness through petrifaction. 

 Species of the following genera may be seen on the slides, viz. : 

 Coscitwdiscus, from very large to small species ; Triceratiunty 

 triangular and square forms ; and a Biddulphia and Cyclotella. 

 The Polycistina and Foraminifera do not require special mention. 

 The reticulation on the Coscinodisci can be made out with a 

 one-sixth objective and good daylight. Another matter of in- 

 terest that this new find revives is the fact that Dr. C. G. Ehren- 

 berg, in his ' Micro-Geologie,' listed many of the living fresh-water 



