I9IS-] 



RODDY— CONCRETIONS IN STREAMS. 



249 



Structure. — Most specimens have as the nucleus a quartz or 

 limestone pebble of the country rock. Near Millersville, where the 

 stream flows for a mile or two parallel to an igneous dyke, the 

 nuclei are diabase pebbles. But some specimens lack the stony 

 nucleus having instead the limy layers concentric around a dark 

 spot which proves upon close examination to be carbonaceous mat- 

 ter resembling nearly structureless peat. Probably this was origi- 

 nally a piece of wood or other vegetable tissue that carbonized a#ter 

 the concretionary laminae had accumulated around it. This sup- 

 position has been verified in a number of cases by finding con- 

 cretions with organic matter as nuclei (see Fig. 2). 



Fig. 2. Sections of a group of the concretions showing the laminae, 

 concentric arrangement of the laminae, the nucleus or nuclear point, and 

 eccentric manner of growth. One-third natural size. The nucleus in the 

 small upper specimen is a small water worn quartz pebble. The larger upper 

 specimen shows where the nucleus was broken out w'hen the section was 

 made. 



The concretions with stony nuclei may always be detected by 

 their higher specific gravity. 



Around the nucleus of a specimen is layer on layer of the limy 

 matter each lamina from one eighth to one fourth of an inch in 



PROC. AMER. PHIL. SOC, LIV. 2X8 Q, PRINTED AUG. lO, I915. 



