Mr. W. J. Sollas on the Genus Catagma. 361 



to wliich they belong in Canada balsam is no doubt true, 

 though even then they leave a thin transparent sheath behind, 

 which, to some extent, maintains the original spicular form : 

 no less true is it, according to Carter, that they break down 

 when mounted dry or enclosed in the kerataceous fibre of a 

 Psammonematous sponge ; but when well cleaned by caustic 

 potash and then mounted in balsam, they last much longer, 

 and if placed in distilled water instead of balsam they will 

 keep for years. I have now before me some calcareous spi- 

 cules which were so prepared ; and they are as perfect to-day, 

 even to their points, as they were when bottled and put away 

 four years ago. Thus there is no antecedent improbabihty 

 about the preservation of calcareous spicules. 



As regards siliceous sponges, we have, 

 as has been said, numberless fossil ex- Fig. 3. 



amples, many of these often existing in 

 a calcareous state ; but it may be as well ^. yfK 



to note that when a siliceous sponge j^|-P| 



becomes calcitized in fossilization, the (JN 



displaced silica is generally to be found \J/ \^ 



somewhere not far off, either in patches 

 in the sponge itself, or in granules or , . , i f^ 



nodules such as flints in the surrounding ^Xssolving Carbonife" 

 matrix, or as chalcedony silicitynig as- rous limestone from 

 sociated calcareous shells, ex. gr. in the Caldon Low, Derby- 

 Lias of the South-Welsh coast, or in shire, (x 140.) 

 minute dispersed crystals of quartz * 



(fig. 3), Devonian and Carboniferous. In compact strata, 

 such as chalk or limestone, it may be taken as an almost in- 

 variable rule that the replacement of organic silica by calcite 

 is always accompanied by a subsequent deposition of the silica 

 in some form or other ; and thus, if one finds flints, chalcedo- 

 nized shells, or minute quartz crystals in such strata, one will 

 naturally look for the remains of the siliceous organisms which 

 supplied them, and one's search will seldom be unsuccessful. 



On the other hand, when one finds large masses of such a 



* Attention was first directed to these crystals by my friend Mr. T. 

 Wardle, F.G.S., in a paper on " Limestone " read before the North-Staf- 

 fordshire Field Club in 1873. They contain numerous irregular internal 

 cavities, and are frequently twinned. Left as an insoluble residue after 

 the solution of the Mountain-Limestone by the great Permian denuda- 

 tion, they have accumulated to form sandstone beds in the red rocks of 

 the Eden valley, to which my attention was directed by Prof. Morris. 

 Similar but much larger crystals (0-02 inch long) are left on dissolving 

 Devonian limestone containing the so-called Stromatopora concentrica, 

 from Kingsteignton, near Teignmouth. These are completely riddled 

 internally and much excavated on their faces externally by irregular 

 cavities. 



