JUNE, 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 barite 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. 
LIMONITE-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 he 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- 
