AGATE. 



[ 21 ] 



AGATTIISTEGIA, 



or honey, or by chemical infiltrations. 

 Nearly all the agates (onyx, sardonyx, &c.) 

 of commerce are thus treated. Carnelian 

 and sard are either burnt or naturally oxi- 

 dized portions of agate. Sometimes agate 

 contains crystals of calcite imbedded in its 

 substance ; and whether coated over, or 

 replaced, these have sometimes given the 

 outlines in the fortification-agates, which 

 are common and elegant objects. Small 

 stalactites, or concentric knobs, of chalce- 

 dony enclosed and invested in the cavities, 

 when cut through, give rise to the pretty 

 eye-agates. Some chalcedonies contain 

 delicate branching filaments of delessite and 

 other minerals, and are known as moss- 

 agates and mocha-stones. Great micro- 

 scopic interest lies in the so-called moss- 

 agates of the south coast of England and 

 the Isle of Wight. These are altered 

 sponge-masses from the chalk and green- 

 sand ; and, with the chert and flints, have 

 been examined by Bowerbank, who sup- 

 posed them to have originated in the at- 

 traction and solidification, by sponges, of 

 silica dissolved in the water of the ancient 

 ocean. The spicula of sponges are com- 

 monly fovmd ; also very frequently the 

 fibres, sometimes in a perfect state of pre- 

 servation, but usually presenting the ap- 

 pearance of having sufiered to a great extent 

 from maceration and disruption of their 

 component parts previous to fossilization. 

 Generally the fibres adhere together in 

 confused masses, presenting a moss-like 

 appearance, with here and there one or two 

 in a better state of preservation ; and occa- 

 sionally, near the outer surface of the mass, 

 small portions of the tissue are found quite 

 perfect ; in other parts all the intermediate 

 states between perfect preservation and 

 nearly complete decomposition may be ob- 

 served. The siliceous matter in which these 

 remains are imbedded, usually presents a 

 clear and frequently a crystalline aspect, 

 while the remains of the organized mass 

 are strongly tinted with colours : bright 

 red, brown, and ochre-yellow prevail ; but 

 occasionally the colours are milk-white, or 

 bright green. Sometimes the interior of 

 the tubular fibre only is filled with ceilour- 

 ing-matter, whilst the sides are semipellucid 

 or of a milky white ; in others the whole of 

 the fibres are impregnated with it. PI. 25. 

 fig. 14 represents sections of a piece of moss- 

 agate (Indian), showing the silicified fibres 

 of sponge «rt, the gemmules h, a separate 

 fibre at c, and spicula at d. Bowerbunk 



has also described an Alcyonitic structure 

 in a moss-agate. 



The supposed vegetable structures in the 

 majority of such agates, described by Tur- 

 pin, Midler, and oihers, have been shown 

 by Goppert to be inorganic products, 

 chiefly dendritic deposits of oxide of iron. 

 His essay contains an elaborate history of 

 the strange notions which have at various 

 times been propoimded concerning these 

 objects. Gergens has imitated the coloured 

 confervoid appearances by decomposing 

 green vitriol with a weak solution of silicate 

 of soda. 



Confervoid bodies, however, have been 

 recognized in the green moss-agate of In- 

 dia ; and a living Conferva was actually 

 found in colloid silica by Roberts and 

 Slack. Schaftiier has described green con- 

 fervoid tissue, referable to Vaucheria, Spiro- 

 gyra, &c., besides spores, in green Indian 

 stones ; and he traced the green colour of 

 the spots in a red opake jasper to Proto- 

 coccus. 



There are two distinct points connected 

 with the presence of these supposed organic 

 remains in agate : one is, whether they 

 really are organic, and the other is whether 

 they are related to the formation of the 

 agate, or merelj^ accidentally present. The 

 first is a very dilficult point ; we have only 

 the microscopic appearance of the bodies 

 under one set of conditions to judge from : 

 this is always very unsatisfactory ; many of 

 the appearances most peculiar to organic 

 bodies, especially when the latter are not 

 connected so as to form a tissue, can be 

 closely imitated by crystallization. Still 

 the mass of evidence is decidedly in favour 

 of the appearances really representing por- 

 tions of sponges and Confervo3. 



BiBL. Bowerbank, Trans. Geol. Soc. (2) 

 vi. 181 ; Ann. N. II. x. (1842), 9 and 84 ; 

 Quart. J. G. S. iv. 319 ; Goeppert, Batishon 

 Flora, 1848, p. 57 ; Brewster, rhil. Mag. 

 (.3) xxii. 213 ; Noggerath, Ilaidinger's Nat. 

 Abhandl. iii. 93, 147, and Neucs Jahrb. f. 

 Min. 1847, 473 ; Gergens, N. Jahrb. f. M. 

 1858,799; Schafiner, Flora, 1859, no. 36; 

 Bisehof, Lehrb. Chcm. Geol. iii. 630 ; 

 Fieusch, Poggendorf's ^«n. cxxii.94; Lange, 

 Halbedelsteine, ^-c'., 1808 ; Hamilton, Q. J. 

 Geol. Soc. iv. 209 ; Billing, Ihe Science of 

 Gems, 1867 ; Zirkel, Beschaffenh. Min. u. 

 Gest. 1873, 108 ; Rupert Jones, Proc. Geol. 

 Assoc, iv. 439 ; Rudler, Po/j. Sci. Rev. xvi. 

 1877, 23. 



AGATIHSTE'GIA, D"Orb.— An order of 



