322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



frequent occurrence and the interest they occasion bring them more 

 attention than is warranted from the standpoint of the geologist, 

 for to him they have little importance and rarely are rich enough 

 in ore or mineral content to be of economic value. In general, con- 

 cretions are more or less rounded or discoidal masses of solid ma- 

 terial accumulated by condensation or segregation of similar mineral 

 substances. They are sometimes rootlike or cylindrical in shape, or 

 they may even resemble great logs of stone. In geology the term 

 is usually restricted to the segi^egation in concentric layers of cal- 

 careous, siliceous, or clayey matter around some nucleus — a leaf, a 

 shell, or a grain of sand — until there results a spherical or rounded, 

 flattened mass, which may vary in size from a pin head to great 

 balls of rock 10 feet or more in diameter. Single concretions are 

 usually spherical, but some become flattened or much curved. Some- 

 times two or more concretions unite in the course of their growth, 

 and again a new concretion may start on the surface of an older 

 one, enlarging layer by layer. This latter type often results in the 

 curious animal-like objects. These are easily understood if studied 

 in connection with associated examples which show that simple 

 rounded nodules with additional layers of growth developing about 

 them grade into the more complicated specimens in which these 

 extra layers have formed in rather symmetrically arranged lobes 

 (pi. 1, figs. 1 to 5, 14). When extra layers are arranged more or 

 less laterally, curious emblematic objects may result (pi. 1, figs. 11 

 to 13). If a shell serves as a nucleus, its whorls may be so covered 

 that a so-called " fossil peanut " results (pi. 1, figs. 6 to 8) . 



Although concretions can be formed by physical means, as, for 

 example, the rounding and enlargement of mud balls, or by organic 

 means, whereby layer after layer of lime is precipitated around some 

 nucleus by the action of plant growth in streams rich in lime, the 

 majority owe their origin to concentration through chemical action, 

 either as precipitates formed while the enclosing rock is being de- 

 posited, or as aggregates in the rock after deposition. Porous rock 

 formations permitting the migration of mineral-bearing solutions 

 allow the formation of concretions by the last method through cemen- 

 tation, long after the deposition of the enclosing rock. Thus, most 

 of the concretions so common in the glacial clays of the Northern 

 States have resulted from the concentration of small amounts of 

 calcium carbonate scattered throughout the clay, with circulating 

 ground water acting as the transporting agency. 



Concretions containing well-preserved fossils were obviously 

 formed at the same time as the enclosing rock, since otherwise the 

 fossils would be at least partially destroyed. The fossils in such 

 concretions are seldom flattened by the pressure of the overlying 



