FURTHER NOTES ON TIDESWELL DALE QUARRIES. 



homogeneous, and hard Diabase, of a black-grey colour, tinged 

 with green ; fracture conchoidal and dull ; and sound when 

 struck metallic. 



2. In each case where I have observed spheroids, the rock 

 containing them has not only occurred near the surface (as in our 

 quarry, where the spheroidiferous variety is found at a higher 

 level, and near the laud surface, and the other variety lower 

 down), but under circumstances that lead to the conviction that 

 this proximity to the surface has obtained for an immense period of 

 time— a period comparable with that during which a considerable 

 depth of the valleys of the district has been excavated. This 

 proximity with the surface means that, for this length of time, the 

 rock has been in close relationship with the gases and moistures 

 that operate from the surface, and which, where the underlying 

 rocks are susceptible to their mechanical and chemical energies, 

 do so mighty a work of rock-disintegration and metamorphosis. 

 Diabase being a complex rock of igneous origin, and containing a 

 large percentage of potassium and sodium salts, is highly 

 susceptible to decomposition or alteration in presence of aerial 

 or humid re-agents. And in every case of Spheroidiferous 

 variety of the above that I have seen, some such process 

 has taken place, — the cores being always of unaltered rock ; 

 but the enveloping shells of a looser texture — so friable, 

 sometimes, as to crumble into a coarse powder between 

 *}->., the fingers, and the colour 



, _a is decidedly brownish. This 

 ggff' change, which is undoubtedly 

 chemical, may often be 

 discerned in the superficial 

 parts of stray exposed blocks 

 of this stone, as for instance, 

 in walls. I found some good 

 examples in the debris of this 

 quarry — blucks of the un- 

 spheroidal variety, which showed 

 signs of having been exposed 



