254 



G. J, HINDE. — 



SILICEOUS ROCK FROM THE 



3i 



(APPENDICE 



ON THE NATURE OF SOME FRAGMENTS OF 



SILICEOUS ROCK FROffl THE BOULDERCUY OFTHE « RQODE KLIF » (RED CLIFF 



on the southern border of the province of Friesland, 

 By George Jennings Hinde. 



PI.ATE VIII. 



The material, sent to me by Dr. H. VAN CAPPELLE Jr, which 

 forms part of the glacial deposit in the boulder clay of the « Roode 

 klif » is a soft white rock, which readily disintegrates into a fine 

 powder. Its présent incohérent condition is probably due to certain 

 changes, which have taken place in it since it was deposiied in the 

 boulderclay ; for in its présent friable state it would not have endured 

 removal and transport from its original position. The material appears 

 to be entirely siliceous, for there is no reaction when it is treated with 

 nitric acid. 



For microscopic examination a small quantity of the material was 

 washed in water, so as to separate the very finest particles, and 

 samples of both the coarser and finer portions were then mounted 

 with Canada balsam in the usual way. 



Under the microscope the material is seen to be nearly entirely 

 composed of entire and fragmentary spicules of siliceous sponges, for 

 the most part free, but in the\:ase of some of the smaller forms partially 

 cemented together in small grains or flakes. The larger spicules yet 

 retain their originally smooth outlines and show but slight traces of 

 corrosive action, but the surfaces of the smaller forms, when examined 

 under high powers, are pittedandcorroded in varying degrees.The spi- 

 cules in ail cases have lost the brilliant glassy aspect, which charac- 

 terizes those of existing sponges and which is still présent in some 

 fossil forms from Tertiary rocks ; they are however much less corro- 

 ded than those in the powder associated with the flints from the Upper 

 Chalk of England. 



Under polarized light between crossed niçois, most of the larger 

 spicules show the lively tints of quartz, whilst the smaller forms and 

 the minute flakes and grains of siliceous material in the deposit are 

 for the most part opaque and thus appear to retain the original 



