l886.] NEW-YORK MICROSCOPICAL SOCIETY. 95 



same nature are yet to be described, which have been observed 

 in the pyritiferous valves of the diatoms of the London clay, 

 whose novel constitution was explained in 1881, by Messrs. 

 Shrubsole and Kitton. They were found only in the lower por- 

 tion of this stratum which underlies the London basin, often at 

 a depth of 360 feet. Their numbers were so great as to be 

 largely concerned in producing the distinct lamination of the 

 clay, on whose planes they appeared as bright shining films. 

 Several species of Coscinodiscus, Triceratium, Arachnoidiscns, 

 Trinacrta, etc., have been identified : 20 genera in all, and 

 nearly 42 species. Count Castracane attributed the formation 

 of their pyritiferous material to an electro-chemical process, 

 and many others have looked upon it as a mere plating " de- 

 posited upon the siliceous skeleton, as in the electro-galvanic 

 deposition of metals." F. Kitton, however, found that, on 

 digestion in weak nitric acid, the entire frustule disappeared, 

 and concluded that the silica had been replaced by a deposit of 

 pyrites. He also noticed the occurrence of small globules of 

 pyrites, Wtjtj of an inch in diameter, resembling pewter balls or 

 marbles. 



I think you will agree with me, on an examination of several 

 preparations of these diatoms here exhibited, that the following 

 points of mineralogical interest are established : — 



First. That the material referred to by all writers under the 

 broad name of pyrites, consists substantially of the single min- 

 eral pyrite. To determine this point I searched for minute 

 cavities in which the substance might have found opportunity to 

 crystallize, and discovered not only minute drusy surfaces, but 

 also little spherules covered by projecting crystals. The glob- 

 ules which Mr. Kitton detected, appear to have been round and 

 smooth, probably concretionary. On those exhibited to-night 

 may be seen triangular faces, which appear to belong to octa- 

 hedra ; these crystals must therefore consist of pyrite. This 

 conclusion will be confirmed by a glance at the slide on which 

 is mounted a crystalline crust from a specimen of fossil fruit, 

 converted into pyrites, from the London clay at the Island of 

 Sheppey. This drusy surface shows distinct, sharp octahedra of 

 larger size, so that this crystalline form probably prevails in the 

 pyrite crystals throughout the London clay. 



Second. The true color of the pyrite films, when examined on 



