50 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
roo miles from land. Farther out than this, in deep water 
soundings have shown the bottom to consist of a widely spread 
deposit of a white, sticky ooze, which when dried resembles chalk 
in appearance and also in composition.* This is a veritable 
limestone now in course of deposition, and is the product of 
minute specks of living jelly, which abstract the carbonate of lime 
from sea water wherewith to form their shells, which, after the 
organisms are dead, are showered down upon the sea bottom. 
These microscopically minute animals are known as F oraminifera, 
and many of the limestones known to geologists have been built 
up almost entirely by their agency. They are, however, by no 
means the only limestone builders. Coral Polyps play a most 
important part in the production of modern ‘limestones, and that 
they have played as important a part as far back as Devonian and 
Carboniferous times is equally certain. Then again we find some 
limestones made up almost entirely of the remains of Encrinites 
or sea-lilies, and of the shells of molluscs. Muddy water is 
absolutely inimical to the life and growth of these limestone 
builders, and we may be quite sure when we find a mass of pure 
or almost pure limestone of organic origin that the deposit was 
formed far away from land, or at any rate in water absolutely free 
from the influence of streams bearing their freight of mud 
seawards. 
Such a mass of limestone ts that of Central and North Derby- 
shire, and we are justified consequently in taking the first step in 
our reconstruction of the physical features of the Lower Car- 
boniferous Period, by assuming that this immensely thick mass of 
almost pure limestone marks the position of an area of deep and 
perfectly clear water. 
When this mass of limestone of the Pennine Range is traced 
along the country to the north, we lose sight of it in the neigh- 
bourhood of Castleton, owing to its disappearance below the 
Yoredale Shales and Millstone Grit. When it is once more 
brought to the surface in the neighbourhood of Skipton, in. 
* In the abyssal depths of the ocean this calcareous ooze is replaced by a 
red clay, the origin of which is at present unknown. 
q 
a 
q 
7 
a a 
