BOULDERCLAY OF THE « ROODE KLIF » 



255 



amorphous or opalized condition of the silica. The occurence in the 

 same deposit of spicules, partly of amorphous and partly of crystalline 

 silica, is a peculiar feature of this material, for as a rule, the spicules in 

 similar material are ail in the same minerai condition, whether amor- 

 phous, chalcedonic or entirely crystalline. It is worthy of note, that, 

 though crystalline, the larger spicules show their forms very clearly 

 when mounted in Canada balsam. 



In addition to definite sponge spicules the finer residue of the 

 material consists partly of amorphous grains, probably due to the 

 decay of the smaller spicules, and partly of minute rounded bodies, 

 which appear to be moulds of the chambers of Foraminifera, which 

 have been infilled with the colloid silica, derived from the sponge 

 spicules, whilst their own calcareous walls have been subsequently 

 dissolved away. The only other recognizable organisms in the material 

 are a few radiolarians and minute fragments of Polyzoa whose tests 

 have been silicified. Some of the silica is likewise in the form of 

 minute dises or globules similar to those described by the writer (i) 

 in the sponge-beds of the Upper Greensand of the south of England. 



The spicules of which the deposit is mainly composed are extremely 

 various in form and represent ail the principal groups of siliceous 

 sponges. The Monactinellidae are the most numerously represented 

 and next corne the Hexactinellidae and the Tetractinellidae, whilst 

 the Lithistidae contributeonly a small proportion of the total number. 

 As a rule the spicules are common, widely distributed forms, and 

 their in.dividual characters are insufficient to allow in most cases of 

 even generic détermination, and it is therefore only practicable to give 

 gênerai indications of their systematic position in the larger 

 groups. 



i . Monactinellidae 



This division is largely represented by straight or curved acerate 

 spicules (Doppelspitçer, Vosmaer) (figs i — 10, 18, 44) either fusiform 

 and gradually tapering to both ends or subcylindrical with abruptly 

 pointed extremities. They are very variable in length and thickness, 

 ranging from o m ,2 5 to i m ,6 in length and from o m ,oi6 to o m ,o58 in 

 thickness. Usually they are smooth, but some are microspined, the 

 spines being either in rings (fig. 10) or irregularly distributed over the 

 surface (fig. 18). curved cylindrical spicules, with rounded ends are 

 fairly abundant (flgs 33-43). Similar forms have been provisionally 



1 1 1 Philosophical Transactions of the Royal Society. — Part II, i885, p. 427. 



