286 The American Naturalist. [April,. 
may also take place in consequence of a displacement of the 
sedimentary carbonate of lime and the animal remains con- 
tained in it.” He further says, “ when the carbonate of lime 
acts as the precipitant to carbonates of magnesia, protoxide of 
iron and silica, in sea water, equivalent quantities of it enter 
into solution again.” All of which can be interpreted as a cor- 
roboration of the view given above. 
However, the exact method of interchange is described, it 
ean hardly be doubted that the pseudomorphism by which 
silica assumes the form of crystallized cale-spar is closely or 
exactly imitated. Von Buch, in his examination of organic 
silicifications, concluded that the soluble silica deposited from 
solutions took the place only of the organic matter, at least at 
first, and that the substitution of the silica for carbonate of lime 
was later, and in this secondary form appeared as warts, con- 
cretionary rings, etc. Alexander Petzholdt, in his examination 
of a silicified belemnite, found that the silicification began on 
the outside, trati ively through the minute tubes 
of its structure to the interior, finally effecting a continuous 
silicification from the inside to the outside. He found the same 
stages shown in oyster shells; and a section in the center of an 
oyster shell contained 51.78 per cent. silica and 47.81 per cent. 
carbonate of lime, with traces of iron oxide, while its exterior 
was entirely silicified. His observations disproved Von Buch’s 
assumption, and established the fact that the waters carrying 
silica directly removed the carbonate of lime and so replaced 
it with opal-material (soluble hydrated quartz), and that no 
warts, concentric or concretionary rings were formed at all. 
Whether the replacement by silica of organic tissues, as the 
structure of wood or the horny apophyses of brachiopods, may 
involve less obvious conditions than those prevalent in the 
more ordinary mineral replacement of calcite or aragonite by 
silica, or not, still the formal character of the substitution is 
similar. Biscbof,in commenting upon this similarity, says, 
“the penetration of the silicic acid in the minute interspaces of 
the fibrous carbonate of lime, as also all the appearances pre- 
sented by the silicified molluscan shells, agree so completely 
with the penetration of the cale-spar by silicious substances, 
