CRITERIA AS TO MODES OF ORIGIN OF SEDIMENTS 361 



Eecently Roddy has made a careful investigation of the lime concretions 

 discovered by him in streams. He shows that in the streams of south- 

 eastern Pennsylvania concretions made chiefly by blue-green algae grow 

 freely where the amount of calcium bicarbonate in the water reaches 300 

 parts per million. The growth takes place only during the warmer 

 months of the year. 16 



In humid climates it is, however, only under exceptional conditions 

 that stream or lake waters could attain the content of calcium bicarbonate 

 which would result in a free growth of such fresh-water limestones. 

 Under conditions such as those of the Old Red Sandstone basins, where 

 the waters came mainly from crystalline rocks, for the formation of broad 

 and abundant nodular limestones, the cornstones of the British geologists, 

 a climate- of at least semi-arid character would seem a necessary postulate. 

 A truly arid climate is, however, not needed; in fact, the absence of 

 gypsum and salt as associated deposits seems to show that aridity was 

 not present. The physical conditions best adapted for the laying down 

 of the cornstones are those of broad, shallow bodies of warm water, lying- 

 in basins beyond the reach of abundant mechanical sediments and largely 

 evaporated during a dry season. 



Not until the Cretaceous were grasses evolved, and trees even now can 

 not grow in compact forests on semi-arid lands. So far as present 

 knowledge goes, there does not seem to have been during the Devonian 

 a binding of the soil by vegetation adequate to hold the upland waste of 

 semi-arid climates. If that be true, there would appear to have been at 

 that time but little distinction between the weathering phenomena of 

 truly desert and of semi-arid uplands- No effective soil mantle would 

 have held organic acids, atmospheric carbon dioxide, and moisture within 

 its pores. As soon as the rocks were broken fine enough, the waters of 

 the rainy season would sweep the debris away. Lichens, it is true, would 

 live on the bare rocks, and some bacteria in the regolith. In the wet 

 Season these lowly agents would carry on, in a measure, chemical decom- 

 position, but they could hardly be counted as binders of the soil. From 

 these barren uplands there would be a graduation of conditions to the flat 

 parts of river basins. Under a semi-arid climate the level of ground 

 water would not sink far below the surface of these flats. There vegeta- 

 tion could flourish and chemical decay go forward the greater part of the 

 year. Oxidation of the soil would become imperfect: grays and greens 

 as subsoil colors would result. These features are seen to be both quali- 

 tatively and quantitatively different from the typical lagoons of desert 

 climates. 



18 Roddy : Concretions in streams formed by the agency of blue-green algae and related 

 plants. Proc. Amer. Phil. Soc, vol. liv, 1915, pp. 246-258. 



