90 ELEMENTARY GEOLOGY 
finally dropping to the bottom. Ona great part of the 
ocean floor there is an ooze or limestone mud (p. 257), 
which is now forming deposits like the chalk, whose 
origin is similar to this. This deposit on the ocean bed 
is called Globigerina ooze. 
In the past geological ages, a species of animal, now 
very rare in the ocean (the crinoid), built limestone 
beds which are known as crinoidal limestones. Accu- 
mulations of shells sometimes gather on the bottom of 
lakes, forming marl, a whitish clay containmg many 
shells. 
These limestones, whatever their origin, are all made 
of carbonate of lime ; and when a drop of hydrochloric 
acid is placed upon them, they effervesce like calcite. 
They are soft rocks, easily dissolved, varying in color 
from pure white to black. There are numerous im: 
purities, and the color is often due to these. One of, 
the most common adulterations is clay, and when this 
is present in high percentage, the rock is called an 
argillaceous limestone, which grades into calcareous — 
clay rock. In them are usually found many fossil 
fragments of shells or corals (Fig. 57); and some of 
the so-called marble, which is used for mantles, etc., 
owes its beauty to these fossils, which are brought out 
distinctly on the polished surface (Fig. 38). 
Silicious Rocks. Other rocks of animal origin are 
less common. In the shallow lakes, and beneath some 
