Juxe, 1906. New Forms of Concretions — Nichols. 35 



actually found for the individual from which the material for the 

 analysis was taken. This discrepancy would be too great were it not 

 for the fact, elsewhere discussed in these papers, that the specific 

 gravity determined for these mineral aggregates is commonly too 

 low owing to air trapped in pores, cracks, etc., which cannot be 

 wholly removed by boiling or by the air pump. If, however, we 

 assume that all the bases except the barite are in the form of silicates 

 which have a density equal to quartz, the calculated density 3.62 is 

 but slightly lower than that before obtained. 



By the method described on page 27, the space occupied by 

 the quartz and barite may be calculated. The calculation so made 

 shows that the quartz occupies 50% of the volume of the concretion 

 and the bar;te 50%. As sand naturally packed generally ncludes 

 about 40% of voids between the grains, it appears as if the barite had 

 crystallized between the grains of sand and very slightly pushed them 

 apart by pressure when growing. Indeed there are in the slide 

 examined, here and there a few evidences of slight pressure upon the 

 cement in the shape of a rise in the order of interference color com- 

 bined with a wavy extinction. These spots however are very few 

 and very small. 



These specimens are, therefore, not concretions in the narrow 

 sense of the term, but crystal aggregates of barite with sand present 

 as a mechanically held impurity. They bear the same relation to the 

 known occurrences of sandstone with barite cement that the sand- 

 calcite crystals of Fontainebleau and Devil Hill do to the sandstones 

 with calcareous cement. 



L1MONITE-SAND CONCRETIONS, SPRING LAKE, MICHIGAN 



These concretions (Museum No. G. 1223, Plate XXIII) were 

 collected at Spring Lake, Michigan, by the author. They occur on 

 the tops of dunes where the sand has been overgrown with grasses and 

 shrubs. In places the vegetation has disappeared and the sand has 

 again begun to move. Thus there are formed shallow pits where the 

 surface has been removed to depths of from an inch or two to five or 

 six feet below the sod. These concretions lie on the surface of these 

 pits in the loose sand. From the shallowness of some of these pits, 

 it is evident that many of the concretions must be formed within a 

 few inches of the original sodded surface of the dune. Inasmuch as 

 in the deeper pits the supply of concretions is not perceptibly greater 

 than in the shallowest of all, it appears that few, if any, of the concre- 



