246 THOMAS C. BROWN 
Two theories have been advanced to explain the origin of these 
odlitic grains. Some hold that they are chemical precipitates 
and that the concentric odlitic structure is produced by successive 
layers of calcareous or siliceous deposit laid down on minute 
fragments of shells, sand grains, etc., in highly calcareous or silice- 
ous waters. An alternative hypothesis, and one which seems more 
probable, is that cellular plants (algae) have extracted lime car- 
bonate from the water, and have built this up into odlite grains with 
a concentric and fibrous radiated structure. All other types of 
odlites are then derived from these original calcareous grains by 
replacement. Such lowly organized plants can live even in hot 
waters, and odlite grains are now forming in springs like those at 
Carlsbad, due to the activity of algae." In 1891 Rothpletz visited 
Salt Lake and made a detailed study of the oodlitic sand now form- 
ing along the shore. He found that odlites dredged up from the 
bottom, which had not been worked over by the waves, were covered 
with a deep bluish-green mass of algae among which he recognized 
Gloeocapsa and Gloeotheca. As a result of his studies he concludes: 
The odlites of the Great Salt Lake are, therefore, indubitably the product 
of lime-secreting fission-algae, and their formation is proceeding day by day. 
... . According to the present stage of my researches, I am inclined to 
believe that at least the majority of the marine calcareous odélites with regular 
zonal and radial structure are of plant origin, the product of microscopically 
small algae of very low rank, capable of secreting lime. 
Another interesting suggestion, particularly when considered 
in connection with the foregoing discussion of the algal origin of 
the conglomerate-like beds, is that made by Seeley in which he 
called attention to the close resemblance of the internodal grains 
of Nullipores to grains of odlite as furnishing a further explanation 
of odlitic texture. These grains show a concentric structure as 
well as a radiated tubular structure, which would favor the recrys-- 
tallization such as commonly occurs.3 
In 1903 G. Linck published the results of his investigations 
t Geikie, Text-Book of Geology, I, tot. 
2 Botanisches Centralblatt, Nr. 35 (1892). Translated in Am. Geol., X (1802), 
279-82. 
3H. G. Seeley, Brit. Assoc. Adv. Sci., Bath (1888), Proc., pp. 674-75. 
