FOSSILS AND PSEUDOMORPHS. 
55 ' 
treating them with sulphuric acid; the oil or sugar is absorbed 
into the more porous layers, and then carbonized by the action 
of the acid. By first boiling agates in solution of sulphate of 
iron (ferrous sulphate) and then exposing them to heat, by which 
peroxide of iron is formed, red varieties may be produced. By 
various other chemical processes the agates may be stained of a 
blue, green, or yellow colour. ( See also p. 113.) 
Illustrations of Fossilisation. iY o. 34. 
A collection of specimens has been made with the view of 
illustrating the various ways in which organic bodies may be 
more or less perfectly preserved as fossils. The series starts 
with some examples of the hard parts of vegetable structures 
capable of preservation, such as the fibro-vascular tissues of 
“ skeleton leaves ” ; and then passes to the hard structures of 
animals , such as the internal pen of the cuttle-fish, and the 
external shells of most mollusca. Certain organisms, however, 
are practically destitute of any structures capable of being per- 
manently preserved, and are consequently represented in the 
rocks only by impressions or casts, as illustrated by the worm- 
tracks and fucoich markings here exhibited. 
Under certain conditions the original organic structure may 
be preserved, either wholly or partially, with little or no altera- 
tion ; as well seen in the insects in amber, and less perfectly 
illustrated in the preservation of vegetable matter in the form 
of peat. The organic structure, in other cases, suffers molecular 
replacement, in whole or in part, by mineral matter, but its 
internal texture is represented more or less perfectly. This is a 
very common mode of fossilisation, and is illustrated by a wide 
range of specimens, showing mineralisation not only by common 
species — such as quartz, calcite and pyrites — but by minerals 
not usually concerned in fossilisation : here, for instance, are 
structures preserved in haematite, galena, sulphur, &c. 
It often happens that while the original organism has been 
wholly removed by decomposition, the external form has been, 
preserved either as an impression or mould , or as a solid cast ; 
while in certain cases, as in some of the “ box-stones ” of the 
Suffolk Crag, both mould and cast may be present. The series 
is brought to a conclusion by a small group of specimens, showing 
how fossils may suffer distortion by mechanical pressure, as 
often seen in rocks in which slaty cleavage is developed. 
Pseudomorpiis. Case No. 37. 
The mineral collection bequeathed by Mr. Ludlam, contained 
a large suite of specimens illustrating the phenomena of 
pseudomorphism, or the replacement of one mineral by another 
with retention of the original form. This collection is here 
