7i8 PRINGIPLES OF STRATIGRAPHY 



by David White (24), by the degree of injury suffered by the 

 perishable parts, such as bark, leaves, etc. Seeds and spores of river 

 plants, such as the water 'ferns or rhizocarps. may also be carried 

 out to sea and buried in normal marine sediments. Their occur- 

 rence, however, always suggests a river-borne origin for the mud 

 in which they are found, as in the case of the Genesee and Portage 

 muds of the Devonic in which occur the spore cases of the water 

 fern Protosalvinia, mingled with marine organisms. 



16. CoNCRKTioNS. Coucrctious are segregations of mineral 

 matter which grow in size by addition externally, internally or in- 

 terstitially. From the point of view of their origin and relation- 

 ship to the enclosing rocks, two types may be distinguished : ( i ) 

 those forming as contemporaneous accumulations, afterward buried 

 by clastic or other strata, and (2) those forming within the strata 

 after their deposition. This second group clearly belongs to the 

 secondary structures of rocks. 



Concretions of calcium carbonate, of barite, of manganese, and 

 concretions composed of fragmental material cemented by phos- 

 phate of lime are among the first group, forming at the present 

 time. The phosphate concretions are most characteristic of the 

 shore zone, while the manganese concretions are common in the 

 deep sea. The latter ( \'Valther-23 : 70/ ) are most abundant in the 

 Pacific between 767 and 8,183 meters, and in the Indian and 

 Antarctic Oceans between 2,926 and 4,754 meters depth. In the 

 Atlantic Ocean they are found between 767 and 5,211 meters depth, 

 chiefly in the neighborhood of volcanic islands. 



These concretions commonly constitute the uppermost layer of 

 the lithosphere in the deep sea, and they are gradually buried by 

 the accumulation of fine muds. Not infrequently they constitute 

 the foundation on which corals or other sedentary benthonic organ- 

 isms gain a foothold, and such a concretion in moderately deep 

 water may serve as the nucleus about which a coral reef is built up. 

 Chemically formed oolites and pisolites should be mentioned under 

 contemporaneous concretions. These have been fully discussed 

 in a preceding chapter. 



The secondarily formed concretions, or those growing within 

 the strata, and therefore of later age, are represented by clay-stone 

 concretions, so characteristic of shale and clay beds, and readily 

 recognized as belonging to this secondary type by the fact that the 

 stratification lines are seen to pass through them. Large examples 

 of these are found in Devonic and later lutaceous deposits. They 

 have not infrequently grown about fossils. Many of these also 

 show the septarium structure as described below. The Losspiippchen 



