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 he so, three 

 suppositions might reasonably be made : 



i. That the material collected as gelatinous phosphate of 

 iron and lime thrown down b}' 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 phosj^hate by the downward leaching of 

 phosphatic 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 comjjound 

 with it during its decay. In process of time the organic matter 

 would disappear and the i^hosphate 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 stoj^page in the 

 deposition of sand, giving the phosphate time to collect and 

 form a comparatively jDure layer. The strong afiinity 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, biit 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 glaucouite. 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 

 rapidl}^ 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 Avaters shown by the sandstone zone above, the 

 phosphatic deposit recommenced. But it was not in quantit}"- 

 enough to form a bed, and only collected in scattered masses, 

 which, suspended above the heavier sand, were mixed only with 



