STATE GEOLOGIST, 87 
Amorphous Calcium Carbonate.—This forms a large class of 
materials of great practical importance. Its variety of composition is very 
great. The most representative materials of this class are chalk and marl. 
The former is a fine light-colored material derived from the remains of 
foraminifera, echinoderms and other marine organisms. Owing to its 
extreme fineness it becomes readily available for chemical reaction. 
Thoughout the chalk-beds flint-stones are found which either have a direct 
organic origin or have been formed by the precipitation of silica. 
The term marl is applied to calcareous materials formed in connection 
with fresh water lakes or basins by the precipitation of lime, obtained by 
the leaching out of limestone rocks. Various theories are advanced in 
regard to the formation of marls, but none of them have been proven 
beyond doubt, yet the chemical theory seems to come closest to the truth. 
The extreme fineness of grain of marls renders them especially suitable 
for cement-making purposes, and hence large manufacturing districts have 
been built up around the great marl areas of the country. In addition to 
the fineness of grain of marls, the fact that their calcium carbonate is 
amorphous is a material factor in bringing about ready chemical com- 
bination. The one great drawback is the extreme irregularity of compo- 
sition, which causes them to show great fluctuations within narrow areas, 
and hence calls for caution and thorough exploration of newly developed 
marl deposits. 
