MINERAL CHARACTERS. 61 
New Zealand, but it seems probable that in older strata they have not resisted the 
destructive influences of fossilization. 
2. Calcisponges, their Mineral Structure, and the changes in it during fossilization.— 
The carbonate of lime forming the spicules of existing calcisponges is beautifully 
clear and transparent in appearance and is doubly refracting in polarized light. It 
has been stated to be very unstable, so that the spicules are readily liable to dis- 
solution, but there is reason to believe that this is not usually the case. Asa 
general rule, the mineral character of fossil calcisponges has been less altered by 
fossilization than that of fossil siliceous Sponges, though owing to the much 
smaller dimensions of the spicules and their close arrangement in the skeletal 
fibres, their distinctive forms have been largely obliterated, even when the fibres 
have retained their form and mineral structure intact. Detached spicules of 
calcisponges have lately been discovered in beds of Tertiary age at St. Erth in 
Cornwall, and at Goes in Holland, retaining the same clear and transparent appear- 
ance as recent spicules; their structure in every respect appears to have remained 
unaffected by the fossilization, and they could not have been distinguished from 
the spicules of existing forms. Again, in some of the calcisponges from Cretaceous 
strata, near Havre, the spicules, though now so closely interwoven together in the 
fibre that they can only be examined in thin microscopic sections, retain the same 
clear, transparent form of the calcite and the same sharp definite outlines as in 
recent forms. The smaller spicules in these Sponges are, in many instances, 
equally as well preserved as the larger, and they show a greater capacity of 
resistance to the destructive effects of fossilization than siliceous spicules of 
similar dimensions. 
In general the spicular fibres of fossil calcisponges retain their original struc- 
ture of carbonate of lime, though the form of the component spicules has to a large 
extent disappeared, and the fibres are now either of crystalline grains of calcite or 
show a finely radiate prismatic structure. This alteration in the fibres is by no 
means uniform, for in the same Sponge the forms of the component spicules may 
be fairly well shown in one portion, whilst in another the fibres only exhibit an 
ageregation of mineral grains. Thus, in the calcisponges from the Upper Chalk of 
the South of England the fibres have an ivory-white, smooth, glistening appear- 
ance, and they retain their outer form very perfectly, but when examined in thin 
sections under the microscope they are seen to be mostly of granular or fibrous 
crystalline calcite, in which the outlines of the spicules have .disappeared or are 
shown very imperfectly. In the calcisponges from the Upper Greensand of Wilt- 
shire the fibres are of a soft, earthy, greyish-white calcite, and in some cases they 
break up into their component spicules. In these the calcite is, in part at least, 
in a granular state, their outlines are either smooth or exhibit an eroded aspect, 
due to some extent to the cementation of grains of the matrix to their surfaces. 
