The Crystalline Cements.—Eckel. 147 
(pozzuolanic) and socalled “natural Portland” cements has 
been recognized by the public. So far as present practice is 
concerned, several fairly well-marked types of cements are 
manufactured, and any attempt to minimize, on purely theor- 
etical grounds, the value of the generally accepted definitions 
of these types would be a serious error. The more purely the- 
. oretical aspects of the question will, if possible, be discussed 
in a future paper. 
The scheme of classification presented below is based pri- 
marily upon the amount of chemical change caused by the 
processes of manufacture and use; and secondarily upon the 
chemical composition of the cement after setting. Though 
not perfect it is believed to be more consistent and 
complete than any hitherto used. The present paper, 
which is a summary of that part of the discussion of greatest 
interest to geologists, will be followed by a more detailed dis- 
cussion of the technology and properties of the various cement- 
ing materials, to be published in a technical journal. It is prob- 
able that some revision of the grouping here given will be both 
necessary and possible before the more detailed paper is issued ; 
and the writer invites discussion and criticism of the material 
here presented. 
CLASSIFICATION OF CEMENTS. 
I. Simple Cements. Includes all those cementing mater- 
ials which are produced by the expulsion of a liquid or gas 
from the raw material; and whose set is due to the simple re- 
absorption of the liquid or gas and a re-assumption of original 
composition. | 
I. a. Hydrate Cements; set due to reabsorption of water. 
I. b. Carbonate Cements; set due to reabsorption of car- 
bon dioxide. 
II. Complex Cements: Includes all those cementing ma- 
terials whose set is due to the formation of new compounds 
during manufacture or use. 
II. a. Silicate Cement: Set due to the formation of sil- 
icates. 
II. b. Oxychloride Cements: Set due to the formation 
of oxychlorides. 
