1893. ] NEW YORK ACADEMY OF SCIENCES. 119 
or oval; and it would seem as if the part of different tinge 
noticeable in some, is rather concentric with the rest than 
forming a core, or filling a cup-shaped hollow. 
The presence of trilobite tests at the centre may indicate 
some form of concretionary or aggregative action, the material 
having collected around these as a nucleus, If this be so, three 
suppositions might reasonably be made : 
1. That the material collected as gelatinous phosphate of 
iron and lime thrown down by ferruginous and calcareous 
waters coming in contact with phophoric acid liberated in the 
decay of organic matter. 
2. That it originally collected as carbonate or iron and lime, 
but was converted into phosphate by the downward leaching of 
Ee pesic waters passing through the sandstones and sandy 
shales, 
3. That it was originally in the form of organic matter which 
was afterwards replaced by phosphate which formed a compound 
with it during its decay. In process of time the organic matter 
would disappear and the phosphate alone be left. The nodular 
form might have been its original structure, or, more probably, 
caused by the strong tendency of phosphatic deposits to assume 
such a shape. 
The first hypothesis supposes that during the deposition of 
the seam in Zone 2, there was for a short time a stoppage in the 
deposition of sand, giving the phosphate time to collect and 
form a comparatively pure layer. The strong affinity of phos- 
phoric acid for organic matter would cause it to collect, especially 
around the bodies of trilobites. The latter may have been also 
the principal sources of phosphorus, but were probably largely 
assisted by other organisms, minute or soft-bodied, and of the 
presence of which we have little direct evidence, except perhaps 
the glauconite. Iron phosphate is one of the most sticky and 
gelatinous precipitates that ever vexed the soul of the chemist, 
and appears to tend strongly to collect together in masses, 
The layer of fine shale next the nodule bed would indicate a 
rapidly decreasing influx of sediment, followed, perhaps, by a 
total cessation immediately afterwards, while the nodules were 
forming. Then there would seem to have been a period when 
the water was muddy and the sediment deposited rapidly, which, 
perhaps, caused the animals supplying the phosphorus to 
forsake that neighborhood. When they came in again with the 
clearer waters shown by the sandstone zone above, the 
phosphatic deposit recommenced. But it was not in quantity 
enough to form a bed, and only collected in scattered masses, 
which, suspended above the heavier sand, were mixed only with 
