THE BUILDING OF THE DERBYSHIRE LIMESTONE. 



219 



that the marks upon the polished surface of the marble 

 must be sections of some predominant fossil which enters largely 

 into the substance of the rock, and a little consideration will show 

 that this must be of cylindrical form. The outlines are, as will be 

 seen, longitudinal, oblique and transverse sections of a cylindrical 

 body, with a transversely corrugated tube running through it. It 

 would not be easy from these marks alone to construct the objects 

 of which they are the sections, but we derive assistance from 

 " weathered " masses of the rock. It is found that when the face of 

 the rock has been exposed for a considerable time, these fossils, 



being of a more resisting nature than the surrounding material— 

 the viatrix — in which they are embedded, stand out in relief. This 

 is due to the manner in which the calcium carbonate is built up in 

 the fossil, and not to any essential difference of chemical composition. 

 Such weathered masses are to be found in many old limestone 

 quarries. There is an excellent case at Monyash, from which the 

 specimen represented in Fig. 2 was obtained. The structure of 

 the cylindrical fossils now becomes clear. No wonder they should 

 have been called "stone lihes," for these long pencils might well 

 be likened to the stem of a plant ; and in specimens in which 



