314 CUTTRISS : NOTES ON THE CAVES OF YORKSHIRE. 
Millstone Grit, which merely forms a cap on the mountains in 
the west. It is, however, to this cap of hard, insoluble rock that 
these mountains owe their preservation. Like a helmet the cap 
sheds the water from the top and protects the soluble rock beneath. 
The whole of the limestone area under consideration can be 
divided into three distinct sections, which for convenience of 
reference may be classified as : — 
1st. The Yoredales, including the rocks of that formation. 
2nd. The Southern Carboniferous or Craven Section, including 
the Carboniferous Limestone lying between the Upper 
and Middle Craven Faults. 
3rd. The Main Carboniferous Section, comprising the remainder 
of the Carboniferous Limestone outcrop, included within 
the imaginary boundary already defined. 
The essential constituent of limestone is carbonate of lime, with 
which is associated in mechanical admixture a greater or less amount 
of earthy impurities. The carbonate is capable of being dissolved 
slightly in water charged with carbonic acid gas. When the water 
is heated and under great pressure it is able to take up a much 
larger proportion of lime, but these conditions do not apjDly in this 
country. Rain water, as it is precipitated from the clouds, absorbs 
a small amount of carbonic acid gas from the atmosphere, and 
gathers still more from decomposing vegetation on the ground. 
Finding its way into the cracks and fissures which are a characteristic 
feature of the rock, it carries off a jDortion as bicarbonate of lime, 
the remaining earthy impurities being washed away in mechanical 
suspension, or settle down in some pool as mud. The cracks into 
which the water first finds its way may be very minute, but particle 
by particle the rock is worn away, a definite line of drainage is formed, 
and ultimately an extensive system of underground watercourses 
and reservoirs created. The procees is very slow, but Nature has 
no need of hurry. The mechanical effect of the running water, 
assisted by the sand and stones carried along with it, adds its 
erosive action to the chemical solution of the rock and the channels 
more rapidly increase in size. In process of time the passages and 
caverns sometimes become too large to be self-supporting, the roof 
