26 CEMENT MATERIALS AND INDUSTRY [bull. 243. 
and pure limestone; (2) pure hard limestone and clay or shale; (3) soft 
chalky limestone and clay; (I) marl and clay; (5) alkali waste and 
clay; (6) .slag and limestone. 
LIMESTONES. 
Limestone is the most important ingredient, in one form or another, 
in a Portland-cement mixture. Limestones of certain types are 
employed in the manufacture of hydraulic limes, natural cements, and 
slag cements. It will thus be seen that limestone is a very important 
constituent of all the cementing materials discussed in this bulletin. 
For this reason it has seemed desirable to discuss in the present 
section the origin, composition, varieties, and chemical and physical 
characters of limestone in general. This has been done in considerable 
detail. The present section will, therefore, serve as an introduction 
to the discussions of both the Portland and natural cements. 
ORIGIN OF LIMESTONES. 
Limestones a have been formed largely by the accumulation at the 
sea bottom of the calcareous remains of such organisms as the foram- 
inifera, corals, and mollusks. Many of the thick and extensive lime- 
stone deposits of the United States were probably marine deposits 
formed in this way. Some of these limestones still show the fossils 
of which they were formed, but in others all trace of organic origin has 
been destroyed by the fine grinding to which the shells and corals 
were subjected before their deposition at the sea bottom. It is prob- 
able also that a large part of the calcium carbonate of these limestones 
was a purely chemical deposit from solution, cementing the shell frag- 
ments together. 
Other limestones, far less extensive, though important in the present 
connection, owe their origin to the indirect action of organisms. The 
" marls," so important to-day as Portland cement materials, fall in this 
class. As the deposits of this class are of limited extent, however, 
their method of origin may be dismissed here, but will be described 
later, on pages 34-36. 
Deposition from solution by purely chemical means has undoubtedly 
given rise to numerous limestone deposits. When this deposition took 
place in caverns or in the open air it gave rise to onyx deposits and 
to the ''travertine marls" of certain localities in Ohio and elsewhere. 
When it took place in isolated portions of the sea through the evapo- 
ration of the sea water it gave rise to the limestone beds which so 
frequently accompany deposits of salt and gypsum. 
a For a more detailed discussion of this subject the reader will do well to consult Chapters of 
Prof. J. F. Kemp's Handbook of Rocks. 
