THE WORK OF GROUND WATER 



77 



Composition of Concretions. — Concretions are seldom of the same composition as 

 the containing rock; those occurring in limestone are apt to be of silica; in clays and 

 shales, of lime or iron carbonate; in sand and sandstones, of iron oxide or lime carbonate. 

 Lime concretions, or clay stones, are probably more abundant than any others. 



When concretions of limestone and iron carbonate (clay ironstones) are much 

 cracked in the interior and the cracks filled with calcite or quartz, they are called 

 septaria (Fig. 56). In sandstone iron concretions of two kinds may occur: " spherical," 

 in which a spherical shell surrounds a core of sand, and " pipestem," which, as the 

 name implies, are cylindrical. The former 

 are probably formed as the result of the 

 chemical change of some iron mineral in 

 the rock, such as pyrite, which renders 

 the latter soluble. After being thus 

 changed, " it spreads outward as a drop 

 of ink does on blotting paper. Evapora- 

 tion takes place around the outer margin 

 of the solution, iron oxide is precipitated, 

 and the first ring or shell is formed." 

 (J. Geikie.) Pipestem concretions are 

 formed where soluble iron compounds are 

 oxidized about the tubes produced by 

 the roots of plants. 



It is probable that certain masses of 

 gravel in southern California which now 

 stand up as hills have been cemented 

 together by a kind of concretionary 

 action. 1 



The flint nodules that are so abundant in the chalk of southeastern England some- 

 times had their beginnings in sponges which secreted a siliceous skeleton, and in other 

 /ossils. Upon this small quantity of silica as a center, other silica taken from the sea 

 water was added to form the nodular flints. Since by a microscopic examination the 

 structure of the chalk in which the nodules lie can be traced, it is evident that the 

 flint nodules were formed in the chalk mud of the ocean floor, rather than on top of 

 these sediments. 



Time of Formation. — Lime concretions or clay stones are formed by the gradual 

 accumulation of lime carbonate, and during their growth they inclose portions of the 

 sediments in which they lie. They are often formed before the rock containing them 

 is hardened (indurated), as is shown by the facts that (1) they are often cut by joints 

 and (2) when they contain fossils, these remains are seldom flattened by the pressure 

 of the overlying rocks as are those in the surrounding shale. Although many of the 

 concretions which occur in sedimentary rocks were formed while they were in an un- 

 consolidated state and before they were deeply buried, there is no doubt that some were 

 formed after the sediments had been consolidated into rock. 



Oolitic Limestone (Greek, oon, egg, and lithos, a stone), so-called be- 

 cause of its resemblance to fish roe, may be almost completely com- 



Fig. 56. — A polished section of a sep- 

 tarium. The white veins are calcite, the 

 darker portions chiefly lime carbonate. 



Arnold, R., — Jour. Geol., 1907, Vol. 15, pp. 560-570, 



