248 THOMAS C. BROWN 
to cause the formation of odlites. Now, since many of the odlite 
layers show no evidence of the presence of animal fossils, it would 
seem to be reasonable to assume that the organic agents were marine 
algae, either of the coralline type, somewhat similar to those which 
abound wherever odlites are known to be forming in the open 
sea today, or minute algae similar to those which are known to be 
active in the formation of odlites in Salt Lake or the hot springs of 
Carlsbad.* 
In spite of recent publications to the contrary,’ the author has 
in his possession material which will, he is convinced, when thor- 
oughly worked up, prove that every occurrence of siliceous odlite 
in the Ordovician rocks of Pennsylvania is due to the direct replace- 
ment of an original calcareous odlite by silica. These siliceous 
odlites occur at many horizons and in many localities. They 
sometimes occupy widely extended layers, and sometimes occur as 
nodules in layers where the surrounding odlites are either calcare- 
ous or changed to dolomite. In every case when found in place, 
their field association and microscopic character is such that they 
could not have formed as direct chemical precipitates. 
ORIGIN OF THE BARRENS SANDSTONE 
As already noted, these sandstones are composed of fine, 
extremely well-rounded sand grains cemented by a calcareous or 
dolomitic cement which easily weathers out and gives rise to a 
thick mantle of residual sand. The sand grains seem to be uni- 
formly rounded regardless of size. A thin section cut from the 
basal member of this series showed grains ranging from 0.17 mm. 
to 0.73 mm. in diameter (see Fig. 7). A sample of the residual 
sand from the middle of the series gave measurements from 0.13 
mm. to 0.71 mm., and a similar sample from the upper beds 
showed diameters from 0.26 mm. to 0.86 mm. 
As is well known, such fine sands could not have been rounded 
to such a degree in water. It seems, therefore, that the origin of 
these sands and sandstones must be assigned to the work of the 
«The author has under way a further investigation of the origin of odlites, but 
the results are not yet available for publication. 
2 Ziegler, Am. Jour. Sci., ath ser., XXXIV (1912), 113-27. 
