CONCRETIONS — BASSLER 325 



entire sphere is divided into seven or more large polygonal areas, 

 each of which is subdivided into about the same number of polygons 

 containing a central region of minute polygons surrounded by a 

 smooth area. This central region exhibits the same arrangement of 

 polygonal areas as the entire sphere and perhaps further subdivisions 

 were continued on a microscopic scale in the smallest polygons. Ad- 

 ditional specimens of this type of concretion, if available for study, 

 would undoubtedly show the various stages of growth and explam 

 the origin of this interesting type. 



Concretions are of economic value as a source of minerals or ores 

 as well as of well-preserved fossils. The calcareous concretions so 

 abundant in the London clay of southeastern England have been 

 much used in the making of cement. Similarly, clay ironstone 

 nodules, termed sphaerosiderites, have been a prolific source of iron 

 ore in various countries. As a source of well-preserved fossils these 

 nodules are supreme, and the splitting open of concretions is a favor- 

 ite sport of the geologist. Sometimes the concretion has formed 

 about a considerable cluster of fossils, but more often only one of 

 many nodules reveals recognizable remains of an animal or plant. 

 Hence, the possibility of discovery of a fossilized complete fish 

 (pi. 2), a spider, a crustacean, or a leaf (pi. 3) provides an element of 

 chance that adds greatly to the interest of collecting. 



Many valleys of the northern United States where the glacial clays 

 are so prevalent, such as that of the Hudson River, have afforded 

 innumerable specimens, no one precisely like another. The bands 

 of flint concretions lying parallel with the chalk strata in the chalk 

 cliffs of England are so conspicuous that they are known to every 

 traveler. The Tertiary sands and clay formations of our Great 

 Plains, as noted before, comprise layers sometimes crowded with 

 enormous spherical forms. Here, too, the concretions often are so 

 abundant as to strengthen the loose sands and clays sufficiently to 

 form a resistant bed which may stand up as a cliff along a stream. 

 The Upper Paleozoic black shales of New York, Ontario, and Ohio 

 often contain large interbedded concretions that have formed about 

 dismembered bones and plates of gigantic fossil fishes described from 

 these areas. Perhaps the most interesting locality of all to the pale- 

 ontologist is along Mazon Creek in Grundy County, 111., where 

 animal and plant remains of many varied types serve as nuclei for the 

 clay-ironstone nodules occurring in the Coal Measures. Fern leaves 

 exquisitely preserved (pi. 3, fig. 3) and in a great variety of forms 

 are the most abundant fossils here, but primitive cockroaches of 

 gigantic size, very ancient horseshoe crabs (pi. 2, fig. 4), and even 

 early forms of fish (pi. 2, fig. 3) sometimes reward and delight the 

 lucky collector. 



