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dony. Floculent masses of green or red oxide or carbonate of 

 iron are disseminated throagh it and give rise to the"tnoss-agates," 

 as they are termed, of the jewellers' shops. 



Here is a very remarkable one in grey chalcedony, and near 

 it is another, not so beautiful, but still more remarkable, a large 

 choanite partially transfarmed into flint. These tentacles of the 

 mythical sea-anemone are the tubules of the sponge, which were 

 probably filled to some extent with the finer mud of the chalk 

 sea, and have still retained some of its colour. This darker part 

 in the centre is the cloaca) cavity. 



But the true " choanites " like the ventriculites have long 

 since disappeared, and no modern representative is left of these 

 curious tubular sponges. 



Hollow Flints. — There are several examples of these 

 exhibited, and they present ma,uy curious problems. They have 

 evidently been formed round a sponge, for the sponge structure 

 is visible on the internal walls. Considering what a fragile 

 substance sponge is, one is surely correct in drawing ^he inference 

 that Ihe first coating of silica round the substance of the sponge 

 must have been a comparatively rapid process. At any rate, it 

 could not have taken place centuries after the formation of the 

 sponge, and when the chalk was to a great degree consolidated. 

 Some of the hollow spherical flints contain when broken open a 

 fine grayish-white dust. On examination under the microscope 

 this dust is found to be composed of the spicules of — not a 

 sponge — unfortunately. Nature in all that concerns this problem 

 seems to take a pleasure in making it perplexing. It is not the 

 spicules of one kind of sponge that is enclosed in this hermetically 

 sealed hollow sphere, but the siliceous remains of several genera 

 and as many species. How were they gathered together and 

 enclosed withiu this stony envelope ? And how was the stony 

 envelope itself formed ? For the silica of any individual sponge 

 can furnish but a very small portion of the flint which has 

 assumed its form. 



It is in these hollow flints that we find the silica passing 

 into the stable crystalline form of rock-crystal. Very often 

 there is an intermediate layer of purer chalcedony on which the 

 clear crystals of quartz have grown. These used to be mounted 

 as jewels and sold as Brighton diamonds, when found at Clifton 

 as Clifton diamonds, and in Ireland, where they are often 

 mounted in b-^g-oak, they are termed Irish diamonds. 



In the production of these hollow flints the process known 

 as dialysis comes probably into play. We owe the discovery of 



