12 FURTHER NOTES ON TIDESWELL DALE QUARRIES. 



for some considerable time. They could be instantly de- 

 detected, by their brownish hue, or by a gentle blow of the 

 hammer — a blow quite insufficient to break the unaltered stone — 

 when a part of the altered material would " shell " off", leaving 

 the sub-angular nucleus of unaltered stone exposed, which, if 

 entirely stripped of its " shell," would be found to approximate 

 in shape to the original block (Fig. 6). 



What this chemical change consists in I cannot say ; but, 

 probably, the change of colour is due to the raising of ferrous into 

 ferric salts. The fact that such chemical change has taken place 

 is beyond dispute, and that is sufficient for us. How the change 

 came about is more readily answered. It was, undoubtedly, the 

 result of the chemical absorption of some constituent or con- 

 stituents of the atmosphere or the vegetable soil, or of both, 

 which is more probable. Whatever this re-agent, or set of re-agents, 

 may be, it must penetrate the rock by means of the joints, for the 

 material is impervious to liquids or gases ; and hence it must operate 

 upon each block at its surface. I have seen a good example to 

 the point in the peculiar ashy Toadstone of Ashover, where the 

 discoloration due to chemical change follows the fissures and 

 leading joints, sometimes for many feet below the natural surface. 

 When a film of altered material is formed upon such a block, all 

 further chemical change will depend upon the perviousness of 

 this film to the re-agent : but as the altered material in our present 

 subject is of so loose a texture, we need not hesitate to conclude 

 that the re-agentwould slowly pass through it to the surface of the 

 unaltered part. But it would greatly retard the work of chemical 

 change. 



From what has been said, it will be difficult to escape the 

 conclusion that the formation of these spheroids is in some way 

 due to this subsequent work of chemical change in the rock : and 

 this conclusion will be strengthened as we proceed. 



3. Is this work of chemical change, invading each block from 

 its superficies, competent, at any stage of its progress, to leave a 

 rounded nucleus of unaltered material ? It is. Imagine a cube 

 of our rock immersed in some re-agent capable of producing a 



