484 



Proceedings of the Royal Physical Society. 



or less flocculent solid particles, partly organic, but to a 

 greater extent mineral in nature. To cause the removal of 

 this solid suspended matter filtration has been resorted to ; 

 the filters employed being made in the usual manner with 

 gravel and coarse and fine sand. When put into use these 

 filters were noticed to become very rapidly covered with a 

 coating of green slimy matter, which in time caused more 

 or less choking of the apparatus, and rendered frequent 

 cleansing necessary. 



The chemical examination of the dried scum or skin 

 which had formed on the surface of the sand disclosed the 

 fact, that, even after the most careful preparatory cleansing of 

 the sample to remove adhering sand, the material was to a 

 very large extent composed of a mineral residue which was 

 insoluble in acids, but was readily volatilised by the addition 

 of hydric fluoride. The microscopic examination of this 

 residue revealed the fact that the material was made up of 

 diatomaceous valves, in a state of almost absolute purity. 



A purified sample of the valves was forwarded to Mr E. 

 Groves of Saltburn, who kindly undertook the microscopic 

 determination of the species, and who reports that the 

 sample consists almost entirely of Fragilaria capucina, 

 Desm., and that "the filaments are matted together in 

 enormous abundance, but when treated with acid they break 

 up completely into single valves." The following is a list of 

 the species identified, all of which, with the exception of the 

 Fragilaria, were present only in very small quantity : — 



Fragilaria capiocina, Desm. 

 Am2)hora ovalis, var. affinis, Kg. 

 Cymhella lanceolata, Ehr. 



,, Cistula, Hemp. 



,, affinis, Kg. 



,, cuspidata, var. anglica, Lg. 

 Encyonema ccespitosum, Kg. 



,, turgidicm, Greg. 



Navicula radiosa, Kg. 



Achnanthes lanceolata, Kg. 



,, minatissima. Kg. 



Cocconeis Pediculus, Ehr. 

 Cymatopleura Solea, Sm. 

 Nitzschia linearis. 



„ sininata, var. tahellaria, 



Gruii. 

 Synedra ulna, var. danica. Kg. 

 Melosira varians, Agh. ? 



The deposit is interesting from the presence of so large a 

 proportion of the Fragilaria capucina, which, whilst it is by 

 no means an uncommon species of Diatom, yet is usually very 

 largely mixed with other species. 



