14 FURTHER NOTES ON TIDESWELL DAI.E QUARRIES. 



I have seen a good example of the above at Mount Sorrel. 

 There in the sub-soil may be observed 

 rounded blocks of granite of various 

 sizes, packed in masses of disin- 

 tegrated granite. Similar examples in 

 other materials frequently occur, one 

 particularly (Fig. 8) was excellent — a 

 cubical lump of sodium had been 

 left in a loosely corked bottle, with 

 F 'g- 8 - this result : — its section presented a 



mass of the oxide, containing a small ball of the metal in the 

 centre. 



4. We now consider the concentric zones of our spheroids. Is 

 this theory of subsequent chemical change competent to account 

 for them ? It is, I think. They may, if this change be complex 

 — the work of many re-agents— be caused by variations in the 

 proportions of these re-agents, due to varying seasons ; hence 

 giving rise to zones in the altered materials of slightly differing 

 textures or proportions of constituents, which, although not 

 obvious at first, may ultimately rupture along the lines of weaker 

 material. 



But a more satisfactory explanation is to be found in the com- 

 bination of the following two circumstances : — The expansion of 

 the altered material, and the resistance offered to this expansion 

 on account of the closely packed condition of the rock, each 

 block having its own expansion resisted by that of its neighbours. 

 That expansion does take place I had ample proof in the 

 vicinity of the quarry, where many of the stray blocks already 

 noticed had a line of more or less distinct cracks round each 

 side, about one inch or more from its edge (Fig. 6, a, a); the rest 

 of the surface being free from them. I can only account for them 

 as the result of unequal expansion between the thicker part of the 

 crust in the vicinity of the angles and the thinner upon the 

 sides. 



When a film of altered material (as a a, Fig. 9 ' is forming upon 

 a body, it is free to expand in a direction away from the latter, 



