26 THE UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 
(not geological), and has been raised to demonstration so far as 
one-half of the area is concerned by a close examination of the 
country. 
The production of spaces capable of transmitting water 
through a dense and compact rock like the average unweathered 
Carboniferous Limestone is affected by the solvent action of 
rain-water, which is always more or less charged with carbonic 
acid. This action is further greatly aided by organic acids 
derived from vegetable matter undergoing decomposition in the 
soil ; and the activity of plants must also augment the quantity 
of carbonic acid available. 
Now in a compact limestone solvent action is limited in the 
main to attacks upon actual surfaces, and the effects will be 
essentially different from those produced on more porous rocks 
of similar composition. 
Such attacks proceed against the upper exposed surface, 
resulting in the fantastically furrowed and weathered forms 
Avhich are so much in favour for the construction of "rockeries"; 
but the}' also proceed downwards along the exceedingly narrow 
joint fissures which traverse all rocks in at least two directions. 
The joints are widened, especially near the surface of the ground 
where the first contact with the acid water takes place, and 
there is also a selective action, some beds resisting solution more 
than others. Flow also occurs along bedding planes, with the effect 
of producing openings following the inclination of the bedding. 
These occasionally assume the dimensions of spacious chambers, 
and they are frequently enlarged by falls of the roof. The lime- 
stone caverns of Craven are sometimes of this character. The 
joint fissures are, however, far more numerous and important, and 
most frequently give rise to ca-verns. 
Where solution is taking place with the greatest freedom 
all the rainfall which escapes re-evajDoration is absorbed into the 
rock, with the frequent result of widening the joints so much at 
the surface as to cause the swallowing up of the soil and the 
production of the well-known " Clints " or "limestone-pavements" 
(See Plate VI.). 
