Mr. J. S. Bowerbank on Moss Agates. 9 



Belfast, in an opposite direction from Donegal Square. Leaving 

 town for some time on the following day, I had not any further op- 

 portunity of witnessing the interesting phenomenon. 



" W. Thompson." 



IV. — On the spongeous origin of Moss Agates and other sili- 

 ceous bodies. By J. S. Bowerbank, Esq., F.G.S. * 

 [With Three Plates ] 

 Ix the course of the last session I had the honour of sub- 

 mitting to the Geological Society a paper u On the structure 

 and origin of the flinty bodies of the chalk and greensand 

 formations of England/' in which I endeavoured to prove that 

 the greater portion of these siliceous masses were derived from 

 the silicification of spongeous bodies which existed at the 

 bottom of the sea at the periods of the deposit of these strata 

 in as great abundance as their recent types are now found in 

 the ocean, both in tropical and temperate latitudes. In my 

 description of the organic contents of the flints and cherts 

 there described, I mentioned the frequent occurrence of spi- 

 cula among these remains. From their appearance in bodies 

 which bore every appearance of being true keratose sponges 

 in which spicula were not at that time thought to exist, I was 

 led to believe that the sponges which had originated these 

 siliceous masses were an order of the tribe differing from our 

 recent keratose types only by the possession of siliceous spi- 

 cula, and therefore, although not absolutely belonging to the 

 same genus as the sponges of commerce, yet so nearly allied 

 to them in every other respect as to leave no reasonable doubt 

 of the true spongeous nature of the fibre that abounds in them. 

 Since that period I have received from my friend Rupert Kirk, 

 Esq., of Sydney, numerous specimens of at least three distinct 

 genera of sponges, and among them many keratose ones, 

 which upon examination with a microscopic power of 500 

 linear, proved to contain siliceous spicula in great quantities. 

 This circumstance induced me to suspect their presence in 

 the sponges of commerce, and upon examining them carefully 

 I detected spicula in each of the two species from the Medi- 

 terranean as well as in that from the West Indian Islands, 

 although, I believe, every author who has hitherto described 

 the sponges of commerce has denied their existence in these 

 bodies. Since the publication of these facts, I have had the 

 opportunity of examining two species of keratose sponges in 

 the collection at the British Museum, which are preserved in 

 spirit in the state in which they were immediately after being 

 taken from their native element, and in both these specimens 

 * Head before the Geological Society of London, April 7, 1811. 



