28 Field Columbian Museum — Geology, Vol. III. 



such a constitution. There are four hypotheses which may account 

 for this excess of calcite: i. 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 



