TRANSFORMATION OF SILICA 149 



consist of opal, but of crystalline silica, devoid of the 

 water which is one of the constituents of opal. The 

 crystalline silica very frequently occurs in the form of 

 slender prisms crowded in radiate growth about a centre 

 (Fig. 42). It is possible that the silica of these solid 

 crystalline spicules is due to subsequent deposition within 

 the cavity of spicules previously corroded. On the other 

 hand, it may be formed by a direct transformation of the 



FIG. 42. Skeletal Remains of Sponges, converted 

 into Fibrous Crystalline Silica : on the right, 

 the head of a three-pronged spicule ; in the 

 middle, part of the shaft of a needle-shaped 

 spicule; on the left, a fragment of the 

 siliceous network of a Hexactinellid sponge 

 ( x 45). 



opal of spicules which have not suffered from solution, 

 though this seems less likely. In the same flint we 

 may find every transition between spicules consisting 

 of opal and others wholly composed of crystalline silica. 



These observations do not advance us very far, but 

 now we will transfer some of the chalky powder to a 

 glass vessel, and pour upon it a quantity of dilute hydro- 

 chloric acid. A brisk effervescence at once assures us that 

 carbonate of lime is in process of solution ; the foramini- 



