26 Field Columbian Museum — Geology, Vol. III. 



The slight flattening of the complete individual as well as the greater 

 flattening of the subordinate spheroids of the head is in the plane of 

 bedding of the surrounding sand. 



The specimens in the possession of the Museum weigh from 45 

 to 952 grams. The diameter of the ball lies between 30 and 70 

 millimeters, and that of the thickest part of the stem between 20 and 

 30 millimeters. The head of the concretion, therefore, varies much 

 more in size than the stem. The stems, however, are very variable 

 in length; the shortest is 55 and the longest 210 millimeters. Two of 

 the specimens are compound, consisting respectively of two and three 

 individuals grown together. 



The specific gravity of the concretions is 2.69, and they are 

 therefore a little denser than the average concretion of this character. 

 Concretions of sand and calcite from Saratoga Springs in the Museum 

 collections have a density of 2.62; those from Fontainebleau of 2.42. 

 The sand-calcite concretions and crystals from Devil Hill, Wyoming, 

 which have been studied by Barbour,* have a specific gravity, as 

 determined by the present writer, of 2.64. According to Dana, the 

 Fontainebleau crystals vary in specific gravity from 2.53 to 2.84.! 

 The great variation in these figures is, however, not to be taken as 

 indicating corresponding variations in the true density of the objects. 

 They rather indicate differences in the methods employed by various 

 experimenters and differences in the shape and size of the pores of 

 different specimens. It is evident that the true specific gravity of a 

 mixture of calcite and quartz cannot be less than 2.65, the specific 

 gravity of the lighter constituent. The very great influence of the 

 character of the pores and of the shape and size of permeable objects 

 of the character of those under consideration are discussed in this paper, 

 page 50. For the reasons there given, the specific gravities of the 

 Salton, Fontainebleau, and Saratoga Springs specimens, determined at 

 the Museum, are probably low, but it is believed only slightly so. 



The carbonate cement of the Salton concretions is soluble rapidly 

 and with brisk effervescence in cold dilute hydrochloric acid, and is 

 therefore essentially calcite. The dissolved cement, however, yie'ds 

 noticeable quantities of iron to chemical tests. 



The sand of the Salton concretions, when cleansed by cold dilute 

 hydrochloric acid, is of a light gray color, subangular, and very fine. 

 It all passes a 60 mesh sieve, 17% is retained upon an 80 mesh, 

 48% additional upon 100 mesh, and 35% passes through a sieve of 100 



♦Bull. Geol. Soc. Am., Vol. XII. p. 165. 

 t Dana : System of Mineralogy, p. 266. 



