180 DR. HINDE AND ME. HOLMES ON SPONGE-REMAINS 



with from 4 to 10 per cent, of carbonate of lime, and it consists 

 chiefly of amorphous and clayey matter and fine mineral particles 

 from the neighbouring land, whilst there are very few siliceous 

 organisms in it. 



The orAj fossil deposit which nearly resembles in character the 

 Oamaru rock is the so-called Radiolarian rock o£ Barbados. In 

 this, however, the radiolaria preponderate, but diatoms also are 

 abundant and many of the forms are, according to Messrs. Grrove 

 and Sturt, identical with those in Oamaru. Some characteristic 

 sponge-spicules are also common to these widely-separated de- 

 posits, but, so far as we are aware, the sponge-remains in the 

 Barbados rock have as yet not been systematically studied. 



Mineral and other conditions oe the Sponge-remains. 



As a rule, in the rock unaltered by heat, but little chemical 

 change appears to have taken place in the siliceous skeletons of 

 which it is composed. The sponge-spicules, radiolaria, and 

 diatoms retain, for the most part, the same smooth, brilliant, 

 glassy appearance as in existing forms. In a few instances, 

 however, this clear glassy aspect is replaced by a dull milk-white 

 tint, and the spicules thus changed are precisely similar in 

 structure to the least-altered fossil forms occurring in the Upper 

 Greensaud Sponge-beds at Warminster, Wiltshire *, and in the 

 Westphaliaa beds of the age of the Upper Chalk described by 

 V. Zittelf. Occasionally also spicules are met with traversed 

 throughout by minute curved cracks or lines like those already J 

 described in certain of the Upper Grreensand beds of this 

 country. But iu both the glassy and milk-white conditions the 

 silica of the spicules retains the colloid or opalized state, and nc 

 instance has been observed in which it has passed into the form 

 of chalcedony, as is generally the case with fossil spicules from 

 the Cretaceous and older rocks. The few instances in which 

 change from the glassy to the milk-white state has taken place 

 tend to show that the unusually perfect condition of preservation 

 is due to the more recent age of the beds and to specially 

 favourable circumstances of f ossilization. It may be said that, as 

 a whole, the condition of these Oamaru spicules differs hardly at 

 all from that of the detached spicules brought up by the dredge 



* Phil. Trans, vol. clxxv. pt. ii. 1885, p. 426. 



t " Ueber Coeloptychiiim," Bay. Akad. d. Wiss. B . xii. Abth. iii. p. 29: 



X Phil. Trans. I. c."p. 426, pi. 40. figs. 8, 9. 



