REPLACEMENT OF OPAL 145 



but this is one of the points on which information is at 

 present lacking. 



That the opal of organic skeletons may enter into 

 solution, so that these may completely dissolve away, has 

 been proved in the most convincing manner. The first 

 complete proof was given in the case cf a sponge, which, 

 on account of the regular cross-like knots formed by the 

 siliceous threads of its skeleton, has received the name 

 of Stauronema. It occurs as a fossil at the base of the 

 chalk, and its skeleton, while agreeing in every essential 

 detail with siliceous sponges belonging to the same group 



PIG 40. A Fragment of the Skeletal Network 

 of an Hexactinellid Sponge (Scleroplegma 

 conica, O.S., after F. E. Schulze), magnified 

 50 diameters. 



(Fig. 40) as the Venus's flower-basket (Hexactinellidse) , 

 yet consists no longer of opal but of carbonate of lime. 

 The natural conclusion to be drawn from this is that the 

 opal of which the skeleton of this sponge was originally 

 composed has been removed in solution and its place 

 taken by the mineral carbonate of lime. This is an 

 instance of a process familiar to mineralogists as pseudo- 

 morphosis. Confirmation is given to this view by the 

 existence of intermediate stages of replacement ; some 

 parts of the skeleton have escaped solution to a greater 

 or less extent, and the residual silica deprived of water 

 may now be recognised under the microscope. 



11 



