The literature on the subject of concretions is somewhat limited in extent, and consists largely of descriptive rather than theoretical matter. It is safe to say, however, that distinct types present different problems for solution, and have resulted from divers combinations of chemical and physical laws. The forces brought to play in the forming of one kind may have played no part in the creation of another. Types vary to such a degree that a valid classification is difficult to prepare. Certain writers have made general classifications with reference to manner of growth; for instance, Dana2 employs the terms "centrifugal" and "centripetal" concretions for growths to and from a center respectively. The latter includes principally concretions of a geodal character. In a similar way the terms "excretions" and "incretions" have been used.3 There can be no doubt as to the occurrence of these two general types, but it has been supposed, in many cases, that concretions have originated only through chemical phenomena. There are exceptions, however, in which certain forms of rounded nodules have been considered as resultant forms of physical forces. Kindle4 accounts for certain concretions of the Chemung by pressure of rising gases of organic origin beneath impervious strata in a semi-plastic state. This idea was suggested by observation of Agassiz and Horsford on "raised hemispherical surfaces" in clayey mud near Cambridge.5 Kindle makes use of this theory to account for a band of undistorted fossils along the vertical and lower horizontal surfaces of certain of ' Published by permission of the Director of the U. S. Geological Survey. 2 J. D. Dana, Manual of Geology, 4th ed., p. 98. 3 J. E. Todd, "Concretions and Their Geological Effects," Bulletin of the Geological Society of America, Vol. XIV, p. 361. 4 E. M. Kindle, "Concretions in Chemung of Southern New York," American Geology, June, 1904.