PLATE 25, 26. 
would be formed, varying in figure and size, according to 
the degree of contraction the substance of the stone might 
be capable of ; which would, in some measure, depend on 
the proportion of its component parts, as well as the state 
and pressure of the surrounding stratmn. These cavities 
would remain empty, or be gradually filled, by infiltration, 
with whatever substance the water, afterwards pervading 
such stratum, contained. The matter thus introduced was 
generally calcareous earth, which we find, lodged in the 
fissures of the ironstone, in the form of spar, or as a soft 
and loose powder. — Such appears to have been the origin 
of the Ludus Helmontii *, and some other varieties of struc- 
ture observable in nodulous iron-ores; and it is highly 
probable, that the cavity in the present nodule was produced 
in a similar manner— At least, it is certain, that it does not 
owe its form to any organic body; notwithstanding, fossils 
of the same kind are preserved in many collections under 
the fanciful titles of petrified icorms and caterpillars. 
* Or, as it is now usually called, the Beautiful specimens of 
this stone are frequently met with in Derbyshire—In some the divisions con- 
sist of a close, white spar, which, when the specimen is polished, has an 
elegant and singular appearance, contiasted with the black, or dark brown 
colour of the ironstone. In others, cavities occur only partially filled with 
crystallized spar, bitumen, and ochre blended together. Some very large 
stones of this kind may be observed, when the water is low, in the bed of 
the small stream, which runs at the back of the grove, belonging to the Old 
Hall at Buxton— -Thty lie in the stratum of shale immediately preceding the 
limestone — Many of them measure two or three yards in diameter. 
