66 
ELEMP^NTS OF GEOLOGY. 
be understood if we examine the mud recently thrown out 
from a pond or canal in which there are shells. If the mud 
be argillaceous, it acquires consistency on drying, and on 
breaking open a portion of it we find that each shell has 
left impressions of its external form. If we then remove 
the shell itself, we find within a solid nucleus of clay, hav¬ 
ing the form of the interior of the shell. This form is often 
very different from that of the outer shell. Thus a cast 
such as a, Fig. 51, commonly called a fossil screw, would 
never be suspected by an inexperienced conchologist to be 
the internal shape of the fossil univalve, Fig. 51. Nor 
should we have imagined at first sight that the shell a and 
the cast 5, Fig. 52, belong to one and the same fossil. The 
reader will observe, in the last-mentioned figure (5, Fig. 52), 
that an empty space shaded dark, which the shell itse^ once 
occupied, now intervenes between the enveloping stone and 
the cast of the smooth interior of the whorls. In such cases 
the shell has been dissolved and the component particles re¬ 
moved by water percolating the rock. If the nucleus were 
taken out, a hollow mould would remain, on which the ex¬ 
ternal form of the shell with its tubercles and, striae, as seen 
in Fig. 52, would be seen embossed. Now if the space 
alluded to between the nucleus and the impression, instead 
of being left empty, has been filled up with calcareous spar, 
flint, pyrites, or other mineral, we then obtain from the 
mould an exact cast both of the external and internal form 
of the original shell. In this manner silicified casts of shells 
have been formed; and if the mud or sand of the nucleus 
happen to be incoherent, or soluble in acid, we can then pro¬ 
cure in flint an empty shell, which in shape, is the exact 
counterpart of the original. This cast may be compared to 
a bronze statue, representing merely the superficial form, 
