146 The American Geologist. 
that the gnathopodites were quite uniform in size and less 
differentiated than in Eurypierus and Pterygotus. 
Yale University Museum, New Haven, Conn., February 5, 
1902. 
THE CLASSIFICATION OF THE CRYSTALLINE 
CEMENTS. 
By Eow1s C. Ecket, N. Y. State Museum, Albany, N. Y. - 
The past few years have been chiefly notable, in economic 
geology, for the number of papers which have been devoted to 
discussions of the occurrence of cement materials or of the tech- 
nology of various classes of cements. In such of these papers 
as were not confined to a consideration of only one type of ce- 
ment, some attempt at classification of the cements has usually 
been made. Two prominent types of these schemes of classi- 
fication may be described as the geological, based upon the 
relations existing between the raw materials of the different 
cements; and the engineering, based almost entirely upon the 
variations in one quality, hydraulicity. Both these plans, if 
carried to their logical conclusions, result in certain incon- 
gruities of grouping. In the following pages an attempt has 
been made to formulate a classification which shall be at once 
rational and practical. In the preparation of a scheme such 
as the following, adapted to the various cements now made, 
it has been necessary to neglect certain relations, more general, 
perhaps, than any which are made use of, but not well adapted 
as bases for a working classification. Prominent among the 
relations which have been neither used nor overlooked, is that 
existing between the members of a series which would begin 
with pure lime and pass through the. hydraulic limes to the 
true cements. Certain practical difficulties prevent the recog- 
nition, in any present day classification, of this relation. For 
in such a theoretically perfect series, Portland cement could 
not be considered to be more than one particular point in the 
transition from pure calcium oxide to pure tri-calcic silicate. 
A statement to that effect, even though justified on theoretical 
grounds, would be extremely inadvisable at present; for it 1s 
only (comparatively) lately that the practical differences be- 
tween this, the most important of our cements, and the slag 
