26 FIELD COLUMBIAN MusEuM—GEOLOGY, Vou. 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 1s.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 frorn 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.T 
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 roo mesh, and 35% passes through a sieve of 100 
* Bull. Geol. Soc. Am., Vol. XII, p. 165. 
+ Dana: System of Mineralogy, p. 266. 
