28 ‘FIELD CoLUMBIAN Musrtum—GeEo .oey, VoL. III. 
Such a constitution. There are four hypotheses which may account 
for this excess of calcite: 1. The concretion may have formed in a 
partially opened crevice; 2. Part of the calcite may be fragmental; 
3. Part of the sand may be impregnated with or replaced by calcite; 
4. The calcite when crystallizing may have exerted pressure upon the 
sand grains and moved them apart. ; 
The first hypothesis, a partially opened fissure, is practically. 
negatived by many conditions and may be dismissed at once. Be- 
tween the other three, microscopic study might discriminate. A 
slide was therefore prepared for this purpose, from a cross section of 
the stem of a concretion. The sand grains in this slide proved to be of 
the usual character of those sands which are derived from acid crystal- 
line rocks. The great majority of the grains were quartz. Partially 
kaolinized feldspars were present in some quantity, also scattered 
fragments of biotite, muscovite, dark amphiboles, and a few grains of 
minerals not readily recognized. Such minerals as garnet, ilmenite, 
magnetite, etc., were completely absent. The grains varied from 
angular to well-rounded, but the greater portion were of a sub-angular 
character. With the exception of the slight kaolinization of the eld- 
spars the minerals of the sand grains were wholly unaltered. The cal- 
cite proved to be wholly in the cement, and the cement contained no 
other mineral than calcite. No alteration of the calcite was observed, 
nor any calcite of fragmental origin, nor did any of it replace sand. 
The calcite was found to occupy more than half the area of the slide, 
the grains of sand seldom touched, but were separated by bands of 
calcite cement, which varied greatly in width. These calcite bands 
were frequently much wider than the diameter of the enclosed grains. 
It appears, therefore, that the calcite in crystallizing has exerted 
sufficient pressure to push apart the sand fragments, although no 
anomalous optical features were noted indicating strain in the cement. 
The cement was in the form of calcite crystals of cross sections com- 
parable in magnitude with those of the sand grains. While many of 
them lay in parallel positions, sufficient data could not be secured 
from a study of the slide to determine whether or not the calcite is in 
the form of radiating crystals or of other regular or irregular aggre- 
gates. 
The concretions in the Museum collections which possess a 
character most resembling the Salton forms in shape and appearance 
are from the two well-known localities: the Paris Basin, and Saratoga 
Springs, New York. The specimen from the Paris Basin which 
appears to possess the most in common with the Salton concretions 
